Spambayes is free. You can download it, set it up, and then throughout the day while your computer is on, it'll sort through your mail and throw out the junk.
Then you can download Spybot Search & Destroy. It's free, too. Just precautionary. And Lavasoft Adaware. Keeps things clean. Microsoft malicious software removal thingy-bopper hasn't really helped me once, but it couldn't hurt either.
If your anti-virus software just shuffles the viruses to your recycle bin (which it probably shouldn't) then it might be wise to dump your recycle bin after each scan.
Don't sign up to win anymore boats or sports cars at the mall anymore, hee hee hee...all they want is your email addy.