Do you have the Windows XP setup CD? you can do a quick repair from the CD. I think my friend had a similar problem and the quick repair might have fixed the problem if I backed up the activation file. Copy c:\windows\system32\wpa.dbl to c:\windows\system32\wpa.dbl.bak and the quick repair just in case. I think the instructions are at
How to Perform a Windows XP Repair Install but you could figure it out once you put in the CD and boot up or computer.