You could take a look in the Registry by typing regedit in the Run open box and hitting Enter. Navigate to H_KEY_LOCAL_MACHINE\Software\Microsoft\WindowsNT\CurrentVersion\ProfileList then in the right hand pane, check each listed under account starting in the left pane with S-1-5-21 until you find the Admin account. it should be top of the list. With luck, you may have a .bak and if you do, rename the entry with no extension to .old then rename the .bak entry by simply removing the extension.
Sadly XP wasn't very good at backing up those Profiles but you may strike lucky. It would be worth importinng that entry form another Administator entry in a different iinstallation of XP. If all else fails, try opening a Command form and type SFC /SCANNOW then hit Enter. You will need your XP CD handy in case the system file checker may want to expand fresh copies of corrupted file.