Insert your OS CD and try this
Press any key on bootup,which will take you to the MAIN MENU
You should have two options
Install and repair
For XP press R key
That should take you to system recovery menu
You can see a command prompt option there,click on it,enter administrator password if needed
Now run following commands one by one and press enter
Restart your PC
for vista and 7, select '' repair your computer ''
and click on command prompt and run the commands as said before
Let us know how it works.