For almost two years Windows Defender (formerly Microsoft Anti Spyware and Giant Antispyware) was available as beta software. This past week the software has gone gold and has been released to the public. If you have no other Antispyware software do yourself a favour and get this. It works half decently and is free! This will also be included in the upcoming version of Windows Vista.
You can download it here http://www.microsoft.com/defender
You also have to run through Windows Genuine Validation processes in order to download.