I use the free cleaner Housecall
http://housecall.antivirus.com/housecall/start_corp.asp
to sweep and clean my machines every so often. The definition files are up to the minute.
I had one virus that would prevent antivirus software from running, but Housecall has two different kernals, so if one doesn't work (crashes/locks/just sits there) try the other kernal. That's how I cleaned it.
It takes a while to sweep a lot of files, but you can specify which drives and folders to sweep, if you have a lot of files, start with the ones that are most likely to be where a virus or spyware might reside such as C:, anywhere you store downloads, emails, browser, temp internet files files first if you need to get back up and running in a shorter time, then do a complete scan overnight. Just check off and blow away everything and anything it finds.