Spyware Terminator is a free anti-spyware, that comes with a highly effective real time guard. Also, it uses only about 2 MB of memory while running in the background. It also has an embedded anti-virus (ClamAV) which you can use if you don’t have any!!
The real time protection of Spyware Terminator has an additional ‘HIPS’ feature, which has not been perfected yet and will drive you mad if you don’t turn it off. So, just keep it off and your computer will continue to be protected as good as ever!!

>Another free anti-spyware with real time protection is Windows Defender. But it’s nowhere as effective as ST, and uses more memory as well (around 6 MB). You can still give it a try from the product catalogue in the Microsoft website.

>You might have heard of Google Pack. it is now offering the Spyware Doctor Starter Edition (with basic real time protection) for free!! You can choose just to download it. It is also not a resource hog as the full edition.

I use this, its not free but works great. Its also sold at best Buy,Wal-mart and a few other places.