Microsoft's new Windows Live Onecare Free Scan
http://safety.live.com/site/en-us/default.htm
is an outstanding program which offers a registry optomizer, defragger and of course, the malware av scanner.
Try it and see.
I also like Microsoft's Baseline Security Analyzer (MBSA)
MBSA is an easy-to-use tool designed for the IT professional that helps small and medium businesses determine their security state in accordance with Microsoft security recommendations and offers specific remediation guidance. Improve your security management process by using MBSA to detect common security misconfigurations and missing security updates on your computer systems.
http://www.microsoft.com/technet/security/...a2/default.mspx
and
Microsoft Malicious Software Removal Tool (Win XP and Win 2000) which updates itself on Tuesdays:
http://www.microsoft.com/security/malwareremove/default.mspx
And of course,
StartUp Inspector (Enables novice and expert users to manage Windows® startup applications):
http://www.windowsstartup.com/
Startup monitor on the same page warns if any program tries to add itself to your startup menu and requires your authorization for it to do so.