You need to get your sysadmin involved probably.

See, it's not enough to just have a USER account on the domain. When an NT computer joins a domain, it also has a MACHINE account which is negotiated with the domain controller. This machine account is part of the security layer that maintains the "trust relationship" between the domain's security database and your PC's security database.

When you disconnect from the domain by switching it to a workgroup, you invalidate the machine account and a new one must be created under a new name (or the old one deleted and re-created). Only someone with the proper privilege (the "add workstations to the domain" privilege) can create a new machine accounts.

This is also a problem when people use DriveImage to create a backup of their OS after they have joined the domain. That machine account's password gets auto-re-negotiated weekly, and if you restore a backup of the OS from more than a week ago, it uses the old invalid password and the account no longer works.