Alternatively, even though (I assume) you have a Windows XP system, you can put a Windows Vista or Windows 7 disk in your computer, boot from it, and hit the "Repair your computer" button on the bottom of the screen. The first thing you should do is click the "startup repair" button, and if that doesnt work, bring up the command prompt and type in "bootrec /fixboot" (without quotes). Remember to remove the DVD before rebooting.
Insert your Windows Vista or Windows 7 disk into your DVD player and restart your computer.
The next screen you see should have the Language, Time and Currency and Keyboard boxes.
In the lower left side there are two options: “What to know before installing Windows” and “Repair your computer.”
Click on “Repair your computer”
Windows will then try to find the installation directory for Vista or Windows 7, depending on what you have.
Next up is a screen with a box called Recovery Options. Click the first option, Startup Repair.
You’ll see a Startup Repair box with the message “Startup Repair is checking your system for problems…”
When it’s done, it will prompt you to reboot. If there is more than one problem it may reboot again to continue repairing.
At this point, you can remove the DVD, reboot and everything should be working.