OK
This is what I suggest
Make an SFC
http://www.vistax64.com/tutorials/66978-system-files-sfc-command.html
If errors are found and not fixed, run the SFC 3 times.
If errors are not corrected. Make a repair install.
http://www.vistax64.com/tutorials/88236-repair-install-vista.html?filter[11]=Performance Maintenance
If that does not help make a clean install.
http://www.vistax64.com/tutorials/68767-clean-install-upgrade-vista.html?ltr=C
Be sure to back up all of your stuff first. The clean install, you will lose your stuff. The repair install, you should not lose your stuff, but back up, stuff may be lost.
You will need your key number. If Vista came with preinstalled the key is on the computer. It is a 25 didget number. If you bought Vista the key is on the sleeve that the CD was in.
Any CD will do, as long as its the same edition (Home Premium) and bit (64 bit) that you have now.
If the OS came with the computer, for the best results, I would contact the computer manufacturer and get an install DVD for cost, usually about $25.00.
If you just want to get this fixed as quickly as possible, without a DVD. You can do what is called a factory reinstall. Give me your computer make and model and I will tell you how.
You will lose your stuff doing a factory reinstall, so back up first. Your computer will go back to the way it was the day that your bought it. No updates, stuff, settings, desktop, etc.
Good luck