Microsoft has a monthly tool called the Malicious Software Removal Tool. To run it click start -> run and type MRT in the box. If your patches are up to date it will say February 2010 in the title bar. Click next and do a full scan.
There are a couple of other free tools you can use other then whatever virus scan software you run. Spybot and Ad-aware are good. Both are available from downloads.com.
To be honest though, I always do a clean install of Windows after I clean a computer. There are some viruses that get deep into Windows and it almost impossible to be sure you got rid of everything. Backup your data to an external drive. While it is still on that drive make sure to run all the tools I mentioned above against the data to make sure there isn't an exe waiting to reinstall itself. Install the Windows boot disc, do a complete format, and reinstall.
I am an I.T. director at a financial institution and pretty good at taking care of these problems and even I wouldn't trust that I got everything out of the computer. There are very sophisticated root kits out there that sit deep in the OS and hide from just about anyone.
Good luck.