Visit an attacker's webpage using Microsoft's browser on Microsoft's operating system, and the attacker can execute arbitrary code on your system with your full privileges. Oh, and thanks to Microsoft innovation - you may remember this from the trial - the browser is integrated with the OS, so reading email from an attacker (opening attachments not necessary) also gives them full access to your machine. MSIE 5.5 is vulnerable, and MSIE 5.01 is vulnerable unless you've installed Internet Explorer 5.01 Service Pack 2. Read the security bulletin and download the patches. Discovery props to Kriptopolis.