I'm sure the above links more than cover what I'm about to say, but I need to get my post count up. So here goes
I use the following applications when removing viruses/spyware;
Kaspersky
NOD32
Spyware Doctor (brilliant program)
Spybot
HijackThis!
and then I install ClamWin for them.
I also find it's best to plug the infected hard drive into another computer, or use a BartPE CD. This way, the files on the infected hard drive won't be locked/in use by the OS when it comes to removing them.
However, in my experience, I find it's usually a better option to simply backup and reinstall the operating system. Simply because scanning for viruses doesn't guarantee to get rid of them all, whereas formatting and reinstalling windows does. Seeing as I supply a 30-day labour guarantee with my work, I may aswell cover myself by guaranteeing to get rid of any malware.
Reinstalling takes roughly the same amount of time (especially once you make your own
custom Windows DVD) as scanning the hard drive and will sort out any other problems the computer may have had.
I make sure to create an image using Acronis though, just incase I need to restore it to exactly the way it was before (malware and all!). I then copy across any of the customers data to the new install and keep the acronis image for 30 days, so they can figure out if anything is missing.