Spybot with the tea timer utility turned on.
Ad-aware as a backup to scan for stuff that spybot may miss

Both of the above are free.

I also have Avast Pro which has some malware checking built in so I'm pretty well covered. You pay a little for it, but its worth it.

If you have Windows Vista, you also have Windows Defender built in, it quietly sits there doing its thing in the background.