UAC locks some administrators

When booting Vista, one admin can log on whereas another cannot. Limited
users can not log in at all. When one cannot log in, the screen goes black
although ctrl+alt+del combination still functions to start task manager or to
log off. When the admin that cannot log in arrives at the black screen, task
manager can run explorer only if it is started with the admin token. The
other admin can log in without problems. Windows also does not detect that
McAfee and Windows Defender are runnning and returns a problem in the
Security Center.

I have tried reseting/renaming the repository and deleting and creating new
accounts. New accounts face the same problem.