If that doesn't work do a repair installation. It accomplishes the same thing but is a little more complicated:
Boot to the XP CD and choose the SECOND repair option, allowing XP to install on top of itself. After completion you'll need to go to Windows Update and download & install all updates (except for hardware & driver related updates, which should never be downloaded from Windows Update - only from the OEM websites). Here is a clear tutorial on how to perform a repair install:http://www.geekstogo.com/forum/topic/138-how-to-repair-windows-xp/
Edited by Allan, 02 June 2011 - 01:55 PM.