In the slight chance you haven't seen this yet, Microsoft just released a Malicious Software Removal Tool targeting some of the most prevalent malicious software. For a list of the tool targets, check its official page.
The tool will be updated on the second Tuesday of every month. If you turn on Automatic Updates, you'll get the latest version automatically. Alternatively, you can always download it from Microsoft Download center.
I would strongly recommend you to run it at least once a month just to cover all your bases.
Official page: http://www.microsoft.com/security/malwareremove/default.mspx
Update: This is different tool than the Microsoft Antispyware. The AntiSpyware is proactive tool that monitors constantly your system. The Malicious Software Removal Tool targets specific pevalent viruses and worms and scans and removes them.
My personal opinion is that this tool will be kept as a way to target specific known threats through Windows Updates. In other words - even if people choose not to run any antivirus and/or antispyware programs on their computers, there is a way for Microsoft to fight virus epidemics.
Btw, this tool is not an antivirus program. It's just a removal tool. You should still run antivirus and antispyware and proactively monitor your computer.