Reasons for Auto logoff

Hello,

Recently, we faced a problem with one of our production server which runs on Windows 2003 and on four processors. The system logged off by itself giving out the message in the spool as "Windows saved user
Server003\admin_user registry while an application or service was still using the registry during log off. The memory used by the user's registry has not been freed. The registry will be unloaded when it is no longer in use" during logoff.
There is no auto logoff functinalities that are set on server. But, we could find that during logoff system was running on low space on C drive. Could this be the reason for logoff ?

Does windows logoff the user automatically, for any reasons ? if so, what are all the reason where it can log you off unless somebody logs into the system and logoff manually.

Any response would be greatly appreciated.

Cheers !!
Vik