I don't know why some people get such a kick out of ruining other people's computers by sending trojans, viruses, and such. Because of these people, everybody needs virus protection. If you do need virus protection for your computer and don't have the bucks to buy it, go to http://free.avg.com/ and download their free program. I have used this free protection for years and have been very satisfied. It will only work on certain operating systems, but most people have these anyway and, as a plus, the program comes in several languages. You can get extra protection by buying the Pro version, but I find the free version to be all I need.
AVG only runs on Windows 2000 SP4 + Update Rollup 1, Windows XP SP2, Windows XP Pro x64 Edition SP1 and Windows Vista/Vista x64 Edition or higher.
Best of all, you have real-time protection. Still, you should schedule a thorough scan at least once a week.