Answers for this are problematic as there are so many different infections and so many differences in what they do on your machine. If it helps, all I can tell you is what I did the last time I had this problem.
1) restart your machine and press F8 then on the screen you get, choose "start the computer in safe mode with networking"
2) if you successfully boot into safe mode then try start/help and support/system restore. If that
won't work then go online and go to "http://housecall.trendmicro.com/"
and run an online scan. Keep a pencil and paper handy to write down infected file names and other info in case you have to remove things manually.
3) If the scan identifies the virus, then run a search on it and you should, as I did, be able to find a site that will give you step by step instructions for removal
NOTE: If you have another machine that can get online I'd use that one for step 3 above