regSeeker, RegCleaner or Ccleaner. All are free and all are good enough for average general purpose user.
It helps to do a little computer cleaning and if you know what you're doing and that's all good.
It helps though if you're having some freezing or BSOD type problems (don't know if they call it BSOD anymore) or some other problems like computer seeming to run slower and all the like.