I had major problems about a month ago when my pc was infected with numerous viruses, spyware, etc when I downloaded a solution for the Rubik's Cube. Due to the number of pop-up ads and sites I was redirected to my pc was barely usable for virtually a fortnight. On the plus side I had plenty of time to do the puzzle!
I spent a few hours each night for over a week trying to get rid of the viruses and using a combination of Zone Alarm, Ad-aware & Spybot I finally managed it. If you don't have a firewall I would definitely install at least the free version of Zone Alarm as even if you are infected you can stop the viruses from doing even more damage while you try to get rid of them with Spybot etc.
I'm not very technical but all of the programs already mentioned, including the AVG software Neil suggested, are really user friendly.
Give them a try, you'll be amazed at how much crap you pick up on your pc.
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