It would be helpful if you provided the OS version for which you have this problem. The more details you provide the better we can asist you. Thanks!
Do a repair with the windows disk. Another way is to use Ultimate Boot CD
Do a repair with the windows disk.
For Windows 2000, a repair can be done by booting off the installation CD and choosing the repair option directly (IIRC).
For XP, you can boot off of the CD and go select to install XP. After you agree to the terms, it should find your current installation and offer to allow you to repair it.
In both cases you may need to install whatever service packs have been made available since the minting of your CD.