Someone called me earlier, because she had a big virus warning on her screen. This was actually a hoax (a web page trying to install malware), and it’s useful to be aware of it so that you know what to recognise.
I identified the website which was responsible, but it’s now been taken down. So, the purpose of this post is to warn you about similar sites rather than this site in particular. I observed the same behaviour in IE8 and Firefox 3.6.12 (on Windows 7), but I haven’t tried any other OS/browser.