There could be some possibilities...The admin acct is hidden on XP when there are other users created on the local PC. It shows up only in safe mode for XP. If the account was delted by chance than you need to log in with domain admin rights(if applicable) or use MMC to login to it and update a user on the pc to admin or recreate the account through MMC. If you're unsure, MMC=Microsoft Management Console. You can type in "MMC" at a run command on an NT PC like XP or win2k and access the application im talking about. From the console menu select add/remove snap in. The select add towards the bottom. You should get a list of options to add. Select Local users and groups. You should be doing this from a networked PC and need to choose to connect to another computer option and put in the name or IP of the PC in question. You should be able to click finish and CLOSE from the windows to get back out. You should then be able to connect to the PCs users console and do any necessary changes.
Another posibility is to check for any services that may be turned off. make sure RPC and any remote management services are turned on in XP.
Hope this helps shed some light.
Jason