I'm guessing a virus gone wrong. Something corrupted your explorer.exe which is a common target for attack in windows. I would say it's best to format the drive and re-install everything. If that is not an option then use something like Trinity Rescue Kit to get your files off and then run anti-virus software on it. It will take some effort on your part to do it right. Once you do that try to boot into safe mode and restore your installation.