Lets rule out malware, go through the First Steps at Removing Malware
Run the System File Checker
Go to the Run box on the Start Menu and type in:
sfc /scannow ( sfc if not reconized) (Note that there is a space between sfc and /scannow)
This command will immediately initiate the Windows File Protection service to scan all protected files and verify their integrity, replacing any files with which it finds a problem.
In all likelihood you will be prompted to insert the Windows XP CD.