I'm pretty sure there's no way you can put the system back on the domain without connecting to the domain controller, from an admin account (both on the local system and the domain controller), and being manually added back in.

There is a password reset mechanism for XP. If I recall correctly, there's a floppy disk image you can download from Microsoft's website that will reset the Administrator account password.

Of course if this is a work system, which would be strongly indicated by the domain bit, I'd just cut your losses and beg the forgiveness of the IT people when next you're in. Find the newest, and lowest paid, member of the staff, and slip them a couple of bucks/quid/<currency> to do it quietly, or at least come up with a plausible excuse as to how it could have spontaneously happened. Mucking about with it further is likely only to make matters worse, and incur greater scorn from the IT people. Best thing to do is admit your mistake, take full responsibility, beg for their help, and hope they don't feel like carrying a grudge.