Server 2003 DC - Can't login in as anyone

Tech-Archive recommends: Fix windows errors by optimizing your registry

I just started an IT job, so please excuse my lack of knowledge
regarding this. I have a server that is running terminal server and
all of a sudden no one can login. I can't even get in as
Administrator. I can boot into safe mode and make the selection for
Domain Controller and then I am able to get in, but then I am not sure
where to even start looking. I did notice that the NET LOGON service
will not start, but I am not sure if that is part of the problem.
When I boot it normally it does say that a service failed to start and
I am wondering if this is it.

Any help would be greatly appreciated.

Thanks

.