Mystery accounts appearing in XP

I’m totally baffled on this. I work for a large organization and we have a very large number of computers. We recently noticed that some mystery accounts have begun appearing on these machines with administrative rights. We’re having some trouble figuring out where these accounts are coming from, so far this is what we know:

1) Appears to be only on laptops
2) consists of 6 lower case followed by 6 upper case
3) removing account from administrative group doesn’t seem to break anything
4) deleting account from machine doesn’t seem to break anything
5) no evidence of account in any log file

Anyone have any suggestions?