The first thing to do is check your system files.
You need to be logged on as administrator.
Click Start, Run, type cmd then click OK to launch the Command Prompt DOS window. Type 'sfc/scannow' (without the quotes). Put your WindowsXP CD ROM in the relevant drive.
Click 'Start' and select 'Run'.
Type: sfc /scannow and click OK
Have your XP installation CD handy, and wait while XP checks itself.