Then, install the hard drive on the old PC and copy the data from C: drive of the new PC to somewhere else and format C:
I don't think repairing will help.
What is the user name? Is it tha Administrator or something else? If it is not the Administrator, if you remember the password you entered in that when you installed XP, you can log in. Press ctrl+alt+del twice at the welcome screen and then you can enter Administrator and password.
Other than that, I don't know a way to recover it.