I've been following and studying computer viruses, vulnerabilities and their malicious exploits in earnest for the past few years, and this has the potential to be the most disruptive vulnerability in a long time. There has been severe security vulnerability discovered in Microsoft's Internet Explorer web browser that allows virtually limitless (in number and severity) attacks to your computer. This vulnerability is exploited simply by visiting web sites. The US Government's Computer Emergency Readiness Team has made announcements and recommendations regarding the vulnerability.
If you are using IE on Windows, I agree with CERT's recommendation and would seriously recommend discontinuing use of Internet Explorer. There are alternatives to IE such as Firefox and Mozilla. Both are very capable browsers, and are not currently vulnerable to severe and known attacks.