Looking at the System Mechanic's page, I'm wondering if they're not over promising their capabilities. A lot of the stuff they do can easily be done with free software, as has been noted, but a few items I've not seen effectively implemented in the past, though I'm willing to admit System Mechanic may have it right. For most of what you want to do, something like CCleaner from Priform would work well as it gives you the ability to do most of the standard cleanup tasks that the disk cleanup utility does, plus a bunch of extras that it doesn't, including cleaning up after programs that leave their garbage files lying around.
Also, while not as much of a concern as it used to be, sometimes cleaning the registry can be helpful, which it sounds like here. Try CCleaner's registry cleanup utility, as that may solve your problem with the missing file, and if you're worried about losing anything, you have the option of backing up the registry items before they get changed.