This can happen when a lot of tweaking of the OS has been done, using optimizers, reg cleaners (besides CCleaner), etc.
If you've used those then unless you can System Restore to before the tweaks were done you're best off doing a Clean Reinstall
then sticking with the tools and methods in the tutorial to maintain a perfect install.
If not then work through these Troubleshooting Steps for Windows 7
leading up to possible Repair Install.