Step one. Download Avast AV or AVG. Both are free, I recommend AVG.
http://free.avg.com install and scan your computer.
For spyware I recommend SpyBot (also free) available at
http://www.spybot.info
Step two. NEVER EVER use Limewire, Kazaa etc. These P2P (peer to peer) programs are extremely dangerous. Apart from the obvious SpyWare and MalWare installed with the programs, it opens up your computer for everyone to see. Kazaa and Limewire use a "Shared Directory" folder, which means anyone can access that folder to take or even to put things in (mostly viruses).
If you are wishing to download movies, music etc. I highly recommend using torrents, or even better buying them. If you really HAVE to download, use torrents.
Torrent programs of note are BitLord and BitComet. I prefer BitLord for it's simplicity, since (IMHO) BitComet is for more advanced.
Third step. Avoid WAREZ sites like the plague, they are literally loaded with Drive-By Downloads (when a file is automatically downloaded onto your pc), SpyWare and MalWare, and of course, the obligatory virus/trojan.
If you need any help with SpyBot or AVG use, just ask. I'm sure there's lots of us that use these two GREAT programs.
Oh. Do you have a firewall installed? This will also help.
Good Luck. Hope you're virus free soon.