Peter,

I could be wrong, but it sounds like when you installed the second copy of
NT that you joined the domain with the original name of the NT workstation.
The trick is to become a member of a workgroup instead.

When an NT worksation or server joins a domain for the first time it creates
a type of trust relationship and an internal password is setup between the
PDC and the domain member.

When you rebooted into the restored version of Windows NT it couldn't logon
because the passwords weren't in synch because you had inadvertently changed
them.

I suggest that you remove the workstation from the domain using server
manager. Reboot and logon to the local machine ..not the domain! Once logged
on, attempt to rejoin the domain by joining a workgroup and then rejoining
the domain. I've never tried this before, but it should work for you.

Good luck,

Nathan