Have you tried the Windows Vista and Windows 7 edition of the Windows Live safety scanner? While Microsoft may have discontinued OneCare products in favor of Microsoft Security Essentials, it still offers a safety scan for Windows 7 & Vista computers.
UPDATE 16th April 2011: Windows Live OneCare Safety Scanner is no longer available for sale and product support has ended. Microsoft has released an on-demand anti-malware scanner called Microsoft Safety Scanner.
The scan consists of:
Protection Scan: Use the free protection scan to check for and remove viruses, spyware, and other potentially unwanted software and to find vulnerabilities in your Internet connection
Tune Up Scan: Is your PC running at its best? Use the free tune up scan to see if you should defragment your hard disk to help improve performance, and then let us do it for you.
Clean Up Scan: Got junk cluttering up your PC? Use the free clean up scan to find and get rid of unwanted stuff on your computer that might be slowing it down.
Check out : Safety Scan for Windows Vista and Windows 7.