I recently ran into the error MMC class not registered error when right clicking Computer and going to manage this also happened when trying to get into the device manager. The internet including Microsoft’s Fix-it Utilities and several forums did not produce a solution. Ironically I had forgotten about a post I made less than 48 hours ago that featured a link to Bill Mullins’ review of Tweaking.com’s repair utility for Windows. In order to fix this error on a known virus free computer, I simply let the program perform a SFC / scannow and went to Start Repairs (skipping the other steps), Custom and selected to register system files. Once I let the utility finish and rebooted, the MMC’s worked great. The utility currenty registers the 1000+ files thus fixing many problems.