logon to win2000 returns to logon screen

I have a major problem. I have installed the MS SUS server onto a NAS box, and had to reboot it for another reason, but now when I try to log on via Terminal services or the console through VNC it starts to log on then logs off again and comes back to the CTRL/ALT/DEL screen. We don’t have any policies in place as yet, and it only affects that machine. It has no CD-ROM so I can’t hack it that way, and when trying to start Telnet session, the session closed straight away with a messages saying the shell process may not have started. I have web admin access to it, and can stop and start processes, and can access all the drives through admin shares but I’ve never seen this before. The logs don’t seem to show any problems.

Can anyone suggest a way to get to the desktop? IF only it was Unix….

Matt