Win NT 4.0 SP6a Sign on disabling:

Here is the Problem:
I have this PC that is in a network, Turned it on but the security portion (where you hit Alt,CTRL,DEL) to sign on won’t come up. The first time this happend was last week, could not figure out the problem so I formatted.
Nowmost of all the clerks here leave at 5pm, but a skeleton crew stays till 7pm. One of the personnel is quite savvy when it comes to tinkering around with registry and blah blah. But before I go pointing fingers I want to make sure this isn’t a commonproblem because so far this seems to be the only machine doing this.
Is there a way to disable this feature?
And please, don’t reply if you have no idea about WinNT. I’m a serious PC Tech looking for a serious sollution to a serious problem.