http://theether.net/kb/100040

If you are using UNDO disks and the computer account password happens to
change on the machine, when you shut down the changes are not saved on the
virtual machine and it reverts to the old settings. The domain controllers
still have the new information. Next time you try to login the information
does not match and you are no longer able to login to the domain.

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This is from the above website
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On a Microsoft Windows NT-based host the computer account passwords are
regularly changed for security purposes. By default, on Windows
2000/2003-based hosts, the computer account password automatically changes
every 30 days.

The security channel's password is stored together with the computer account
on the primary domain controller (PDC), and is replicated to all backup
domain controllers (BDCs). The password is also in LSA secret $MACHINE.ACC
of the host .

If the password is not changed for "MaximumPasswordAge" days the machine
account becomes invalid, denying domain logon.

If a machine is reverted to a previous snapshot the secure channel password
on the host could differ from the copy held by domain controllers, denying
domain logon.

Resolution:
Disable computer account password changes on the affected host and rejoin
the domain:

HKLM\SYSTEM\CurrentControlSet\Services\NetLogon\Parameters
"DisablePasswordChange"=dword:00000001