put it back in the original PC, and see if you still get the same error
if you do, that means the partition is still there
if that's the case, put in your XP CD, boot up with it, and go to the recovery console
once at the command line, after you type the admin password (blank by default), type the folowing two lines:
del C:\windows\system32\config\system \q
copy C:\windows\repair\system C:\windows\system32\config\system
then exit (by typing "exit") and restart without the CD. it should boot afterwards.