I've found that a one-two punch of Lavasoft's AdAware and Spybot Search and Destroy work really really well. You just need to make sure you have the newest reference files. There is a piece of software out there called Spyware Nuker. Do not, I repeat DO NOT install this as it is spyware itself. I've noticed most of our infections at work are caused by user error and/or downloads of software (P2P software like iMesh). You might also have to run them several times to make sure the infection's gone.