Just use an Internet-based service so that you can chat from a Web page without having to install any software, which might be blocked by a firewall. I tested two such services: Meebo at www.meebo.com and KoolIM at www.koolim.com. Both are free.

These services let you simultaneously log in to multiple IM accounts — and communicate with people with various services. If you have a friend who uses Yahoo Messenger, for example, and another who likes MSN Messenger, you can chat with either.

Another plus: Meebo and KoolIM are far less vulnerable to viruses than downloadable applications. They’re also more efficient, saving users the hassle of installing multiple programs on a computer. This is especially handy for people with old computers that slow down when running several applications.

Meebo has a well-designed, sleek interface that makes it appealing to even the least tech savvy. From its home page, you simply sign in for different IM services—MSN Messenger, Yahoo Messenger, GTalk (or Jabber) and AIM (or ICQ). Your buddy list will be combined automatically. You don’t have to register, but if you do, you get perks such as a single sign-on for all of your accounts, and the ability to share files, save chat logs and store conversations.

I tried Meebo on my work Windows PC and my iBook at home, and it worked well on both. To start chatting, you just log in to any of the IM services by entering the screen name and password you already have with a service, or by picking a new name, password and services. Your buddy list will appear in a window on the right side of the page, with each name marked by an icon denoting the service the person uses. Once in your buddy list, you can add or delete a contact, message or join a group chat.

The service also offers MeeboMe, a way to chat from your own Web site. Bloggers can use this to start a conversation with site visitors who log in to their Meebo account. It also shows how many users are on the site at any time.

But Meebo, like most such Web services, lacks the ability for audio and video chats — basic features on installed IM software. If you’re used to plugging in a Webcam and headgear to chat with friends, these services might not be for you. Meebo also doesn’t have the standard send button that’s on installed IM software. Instead, you press a key on your keyboard.

Sharing files using Meebo is tricky, too. Only Meebo account holders can send files; although anyone can receive them. When I sent a PDF file to my friend on her installed MSN Messenger, she received a message and a link that rerouted her to Meebo’s home page with no further guidance. When I sent her the same file after she logged in to her MSN using Meebo, she could open the file.

KoolIM also connects users to multiple IM accounts from one Web page. It gives you the option to create an online account, so you don’t need separate log-ins for each IM service you’ll encounter.

You can select an embedded or a pop-up version of the service. The embedded version shows your buddy list and messages as windows tied to a Web page, while the pop-up version lets you move the windows around the screen. You can choose to display all of your buddies or only the ones online. In your buddy-list window you can add and delete names and log in to or out of a service.

I didn’t like having text ads appear at the bottom of the IM window in KoolIM, or the fact that, unlike Meebo, there isn’t a time stamp on messages to tell you when they arrived. It also doesn’t allow you to send files, as Meebo does. And it doesn’t let you save conversations or chat logs. KoolIM’s lack of some of these basic features might make it less appealing to consumers, especially those who ponder swapping it with their desktop software.

I preferred Meebo, but both services are good alternatives to installed IM, if you’re willing to do without audio and video features.

Walt Mossberg is on vacation. Find all my columns and videos online free at the new All Things Digital Web site, http://walt.allthingsd.com.

• Email me at Sarmad.Ali@wsj.com.

These creeps now find it easier than ever to defraud people, steal their identities and blast them with unwanted or false advertising. They use the Web as a pathway to infect computers, corrupt data and take over others’ machines.

Security software can help block this wave of woe. But it would be better to know in advance if a Web site that comes up in a search result, or one you arrived at through other means, is harboring malicious software, or perpetrating scams, or generating spam and unwanted pop-ups. It might also be nice to know if a site with an innocuous name contains pornography, hate speech or other content that might be offensive to you.

I’ve been testing two services that aim to provide such advance notice of bad or offensive sites. The services, Scandoo and SiteAdvisor, take different approaches to the task and offer different features. But both instantly mark up a search-result page, and label the links that might be dangerous.

Both services are free of charge, and each works on both Windows and Macintosh computers, and in multiple Web browsers. On balance, I prefer SiteAdvisor, though Scandoo has a couple of things SiteAdvisor lacks.

Scandoo, still in beta, or test, phase, is from a company called ScanSafe, which provides site-scanning and security services for corporations. SiteAdvisor was founded by some engineers from MIT and was recently bought by McAfee, the big computer-security firm.

SiteAdvisor works via a software plug-in that you download and install. The plug-in, available at www.siteadvisor.com, modifies either the Internet Explorer browser for Windows, or the Firefox browser for Windows, Macintosh and Linux, so the browser can identify bad Web sites. SiteAdvisor works with the Google, Yahoo and MSN search engines.

Scandoo requires no software downloads and works with more browsers than SiteAdvisor does. But it requires you to enter a search term at its Web page, www.scandoo.com, rather than at the home page or search box of your favorite search engine. It then transfers to the search engine you choose and modifies the results page to identify sites that may be troublesome. It now works only with Google or MSN.

There are some other major differences between the two. Scandoo scans Web pages on the fly to look for bad stuff. SiteAdvisor matches Web sites against a database it has compiled about content. Scandoo works only on pure search results, not the ads alongside the results. SiteAdvisor rates the results and the ads, which often are more dangerous.

In addition, because it is built into the browser, SiteAdvisor can rate any site you are visiting, not just sites listed in search results. SiteAdvisor places a small, unobtrusive icon in your browser. The icon is green if you are on a Web page it considers safe and honest. It turns red if it regards the site as dangerous.

Scandoo works only on search results pages. But it has a function SiteAdvisor lacks. It can rate pages for offensive content, while SiteAdvisor focuses just on the presence of malicious software, or invasive advertising techniques. Scandoo allows you to specify which kinds of content you want flagged, including pornography, hate speech and gambling.

SiteAdvisor also flags sites it regards as perpetrating scams, like charging people for software that actually is free. But in my tests, it ignored some other scams, such as offers for pills that magically enlarge body parts.

In my tests, SiteAdvisor consistently flagged more Web sites as bad than Scandoo did. When I searched for “Free iPods” in Google, Scandoo gave all the regular search results a green check mark, meaning OK. SiteAdvisor marked the first regular result in red and gave it an “X,” meaning trouble. It also marked most of the ads in red and gave them “X’s.”

This is partly due to different techniques they use. Scandoo claims its real-time scanning can uncover bad sites SiteAdvisor might miss. SiteAdvisor claims its database is more comprehensive.

Another reason for the disparity is that SiteAdvisor isn’t just looking for viruses or spyware. It uses test computers to see if sites are likely to generate what it calls “spammy” email or pop-up ads. If they do, the sites get flagged.

Some might regard SiteAdvisor’s filters as too aggressive, but, unlike Scandoo, it gives a detailed explanation for each rating. The explanations I saw made sense. For the free iPods site SiteAdvisor flagged, it explained: “After entering our e-mail address on this site, we received 11 e-mails per week. They were very spammy.” It even showed some test emails.

Both services are very helpful. You might want to use Scandoo if you’re concerned about offensive content. But for flagging malicious software and invasive advertising, SiteAdvisor is more comprehensive and tougher.

• Email me at mossberg@wsj.com.