Two programs that every Windows user needs on their machine:
Lavasoft's AdAware Free for personal use and I've found it a little more updated on bad things people are trying to put on my computer.
Spybot - Search and Destroy Totally free and very good.
I like to run AdAware once every two weeks and Spybot once a month. They take a while to work, since they go through every file on your computer and all the entries in the registry. You can set them to work faster but I feel like thorough is better. Maybe if you ran them every day it'd be good to run them in smart mode.