Before going to the considerable effort to thoroughly scan and purge your system of any lurking malware, i recommend first doing a thorough cleaning by using windows own disk cleanup tools, your browsers' temp/cookies/history/cache cleanup utilities, and then finally a rigorous scrubbing and optimizing with CCleaner (http://www.piriform.com/ccleaner/download). these steps will make it much easier and faster for your malware scanners to run and efficiently complete their detection, identification, and removal operations. http://www.piriform.com/ccleaner/download
another thing to consider doing as a part of a comprehensive system cleaning would be to delete all system restore points (malware & viruses often harbor stealthily in there and get back into your system that way after being initially removed), turn off system restore (this completely flushes all the restore point cache/history), and afterward re-enable system restore so that it will once again begin creating automatic restore points for your system - it can be quite useful as long as your restore points are not infected.
another thing to consider doing as a part of a comprehensive system cleaning would be to delete all system restore points (malware & viruses often harbor stealthily in there and get back into your system that way after being initially removed), turn off system restore (this completely flushes all the restore point cache/history), and afterward re-enable system restore so that it will once again begin creating automatic restore points for your system - it can be quite useful as long as your restore points are not infected.