If you are running Windows XP:
Put your Windows CD into the drive, make sure to run the installation. After some time a screen should appear that will offer you to install Windows XP, open the repair console or abort. Open the repair console and once you are inside, enter "chksdk /p". After it is done, enter "fixboot". Finally enter "exit". After the reboot, it should work. If it didn't work, repeat everything until you get into the recovery console. This time type "chkdsk /r" and after it is done "exit". Be aware that chkdsk (especially with /r) will most probably some time to scan and fix your HDD.
I'm not entirely sure how it works in Windows 7, but I'm pretty sure it's very similar to get into the recovery console.