Try doing a repair install:-

How to perform an in-place upgrade (reinstallation) of Windows
http://support.microsoft.com/default...b;en-us;315341

These sites may also be of some use:-

How to Troubleshoot Registry Corruption Issues
http://support.microsoft.com/? id=822705


How to Recover from a Corrupted Registry that Prevents Windows XP from
Starting
http://support.microsoft.com/default...b;en-us;307545


How to back up, edit, and restore the registry in Windows XP and
Windows Server 2003
http://support.microsoft.com/default...b;en-us;322756