A bit of a long shot but did you format the drive and then reinstall XP or did you let XP format it as it was installing ?
I have found this in microsofts knowledge base :
SYMPTOMS
After you install Windows XP, you may observe a "Previous Operating System on Drive Letter:" entry on the Boot menu.
CAUSE
This problem can occur if your hard-disk partition had been created and formatted before you installed Windows XP.
RESOLUTION
To work around this problem, modify the Boot.ini file to remove this entry.