Repairing Windows
1. Insert the Windows install disk for your version of Windows and reboot the PC. With the computer unable to find the operating system on the hard drive, it may skip automatically to the next possible boot device, which should be the CD or DVD-ROM drive. Some systems may be configured to boot to that source first, and others may not be configured to roll over to the next device at all. When in doubt, there should be an option to select boot device when the computer is first powered on (the F12 key on most Dell computers) and you can boot straight to the optical drive from there.
2. Press any key when prompted to boot to the install disk, then follow the on-screen instructions. You'll want to try to perform a repair on the existing Windows install, which you will get to by first selecting a new install of Windows.

3. Select to install a new operating system, and the install package will look to see if Windows is already installed. If it finds the existing install, the installer should give you a chance to perform a repair of what's currently on the hard drive, replacing corrupted files with clean ones from the install disk.
4. If the Windows installer does not detect an existing Windows install on the hard drive, then the hard drive itself may be at fault.
5. Replace the hard drive and repeat the first few steps to install Windows from scratch. The original hard drive is likely corrupted or otherwise broken, and may not be recoverable at all, outside of an expensive, third-party data retrieval service.