Although limited in functionality, you can also install and use Comodo's BOClean (
http://www.comodo.com).. It's free and is a resident scanner. I have it installed on my computer, and it works fine in the background. It's just another layer of protection.
I also heard very high regards for Threatfire by PC Tools (
http://www.pctools.com) as some have mentioned above. I would use that, but there is a bug in the program itself that doesn't let Windows Backup in Vista run while it's running in the background.. It kills the process so the backup fails.. Other than that, ThreatFire is another great behaviour blocker.
I would avoid Spybot. It was good in its day, but is no longer effective, in my opinion. I think of A-Squared as another Ad-Aware 2007, and heard mixed reviews on Spyware Terminator. I'd get the free version of Spyware Doctor from google pack before any of the above..