It's hard to truely "clean" a hard drive off. Even if you reinstall windows the old data will not be erased, your OS will simply not look for it, i.e. it will be lost. But for what you want, I would suggest running a disk defrag and disk clean up first. A lot of times that will clean up your hard drive to a more "runable" state. Then if that doesn't work you can try reinstalling the OS to see if that can help. Another option to get an even cleaner hard drive is to install Linux and have it format the drive during install, then reinstall windows.