SpyBot Seek & Destroy (free AntiSpyware app) is currently on my list of the best free security apps for Windows, and it has been for quite a while. It runs on Vista and XP, it is free, and it gets the job done.
While performing routine maintenance on my home PC today, I was notified of a new version of SpyBot and I allowed the updater to download and install it. I was a little leery at a new version because the existing version has caused no problems and you typically expect that a newer version is going to do some things better, but more importantly, do some things worse.
Some major highlights of the new product include improved immunization, new integration with IE, new integration with Windows Security Center, and a slight update to the user interface.
I highly recommend installing this version.