Please clarify, is the error in XP or W7 and are these completely separate machines or are you dual booting? If the error is in W7 then , when you transferred the files did you use Windows 7 Easy Transfer? If you did use Easy Transfer, what drive letter was your XP on and what drive letter is W7 on (W7 tends to rename the drive to C: and if you transferred from a diff drive letter it may be pointed to the old drive letter)
If the issue is on the old XP: The easiest recommendation to suggest would be to try a system restore to an earlier point prior to the error. Another suggestion would be to log on as an administrator and delete and recreate the erroneous ID, however all settings would likely be lost as well.