The Department of Homeland Security's U.S. Computer Emergency Readiness Team touched off a storm this week when it recommended for security reasons using browsers other than Microsoft Corp.'s Internet Explorer.

The Microsoft browser, the government warned, cannot protect against vulnerabilities in its Internet Information Services (IIS) 5 server programs, which a team of hackers allegedly based in Russia has exploited with a Java script that is appended to Web sites.

The particular virus initiated this week inserts Java script into certain Web sites. When users visit those sites, it initiates pop-up ads on home and office computers, and allows keystroke analysis of user information. The target is believed to be credit card numbers. CERT estimated that as many as tens of thousands of Web sites may be affected.

CERT said vulnerabilities in IIS and IE could include MIME-type determination, the DHTML object model, the IE domain/zone security model and ActiveX scripts. Alternative browsers such as Mozilla or Netscape may not protect users, the agency warned, if those browsers invoke ActiveX control or HTML rendering engines.

The only defense may be completely disabling scripting and ActiveX controls.

Microsoft said earlier in the week it is working with law enforcement officials to identify the source of the latest Internet virus.

The respected research center was among security groups recommending other browsers as long as a key vulnerability in IE remained unfixed, leaving it capable of running malicious code that's been hidden at a number of popular Web sites.

It took a week for Microsoft to issue the update, which does not fix the flaw entirely but disables a hacker's ability to deliver malicious code with it. Ullrich said the update appeared to eliminate any immediate need to switch browsers, which can cause problems of its own.

The flaw had allowed a computer virus to spread through a new technique that converted popular Web sites into virus transmitters. That infection was designed to steal valuable information as Web users typed it into their computers -- passwords and the like.

And this week, researchers discovered another password-stealing program hidden behind pop-up ads. A repair for the flaw enabling that Trojan infection was issued in April, many users had yet to patch their systems.