now you have had a virus.
it probably infected your system from one of a few sources.
1) browser exploit
most common browsers today are IE 6, IE7, FireFox, Safari. if you use one of these browsers, they are very popular for a website of questionable intent to create pages that exploit this. MS fixes more problems than Mozilla does, but MS does it slower than Mozilla. generally when a problem is found in a mozilla browser, it gets fixxed within days to a few weeks. but not all of them get fixxed. for MS while more problems do get fixxed than is even found, its usually few weeks to a few months for a fix.
solution: do not use any MS or Mozilla based browser. find an alternative.
2) java exploit
most people do not realize this, but java is actually a programming language. while it was designed to function inside a web browser, it also operates independantly of the browser. any browser that advertises complete java compatability/implementation is simply telling you that they will allow a web page to access java outside the scope of the browser. easiest browsers to do this with is any Mozilla browser. it is also easy to do this with IE, but much easier in mozilla based browsers. if you use a mozilla based browser, you are more vulnerable than anyone else.
solution: get third-party plugins to restrict/prohibit java functionality in your browser, or use a browser that does not offer complete java compliance.
3) program you downloaded
most prominent in torrent files and such. you use/download these, thats a risk you take.