How to repair Windows XP

If Windows XP is behaving oddly, or you cannot boot the system or log in, the issues may be resolved by performing a repair installation.

Solution
Unfortunately, a repair installation can only be carried out if you have a Windows XP installation CD.

- Insert the Windows installation CD in the drive and start the computer.
- When prompted to do so, press a key to start your computer from the Windows XP CD.
- When the message To setup Windows XP now appears, press Enter.
- When the Welcome to Setup screen appears, press Enter.
- When the Windows XP Licensing Agreement appears, press F8 to agree.
- Make sure that your current installation of Windows XP is selected, then press R to repair Windows XP.
- Follow the instructions to complete the repair installation.

(source: http://www.the-pc-guru.com/How_to_repair_Windows.php#body)