In addition to maintaining a proper antivirus sw on you PC you should also cover the anti spyware segment.
I am using 2 different pieces of software for this segment, Lavasoft Ad-Aware is one of these. (Spybot is the other).
It is however not enough to just install these, you also need to update the definitions and scan your system at regular intervals. It is recommended to do such updates and scans at least once a Month.
Some external Reviews:
You can also read more about the product on the Lavasoft Ad-Aware Free website: http://tinyurl.com/2fexbw