I run both Adaware and Spybot on a monthly basis, and keep them current every time I run them. Even though I have a double firewall (hardware router and XP SP2 software), I still occasionally find stuff that isn't supposed to be there. I just blame my wife, who also uses "my" machine when I'm not around...
I am also in the Spybot & AdAware camp. I have also started using a third app recently called Spyware Doctor as it seems to pick up a few items that are missed by the other two.
I'm a tech - I do this for a living. Get both. Spybot has something called "immunizations". Make sure to turn it on. It'll flat-out prevent your computer from being infected by a good chunk of spyware.
You should note that Ad-aware Personal is only free for home usage, not workplace.