windows 2003 server reboot loop

Hi

We're Running Windows 2003 server SP1 with SQL server SP4 Beta and Backup
exec 9.1

we seem to be in an endless loop with the server rebooting at the point at
which the login box would normally appear, If we set it to last known good it
boots succesfully. but then once it is rebooted again, it goes back into this
loop.

We cant see that we've installed anything to cause the loop, although the
server hasn't been rebooted for a month or so, so it's possible that
something could have been installed that didn't require a reboot.

We're really worried about installing any patches incase we cause more
problems, eg if we installed sp4 full for sql then had to use last known good
after it entered the reboot loop then I would think that restoring some of
the registry back to the previous version while leaving the service pack
installed could cause problems?

one of the symptoms seems to be that if we restart it, and it enters this
loop, then when we bring it back on line it tells us in event viewer that the
server was not shut down correctly, so the problem could be with shutdown
rather than startup?

If anyone has any ideas to further troubleshoot we'd be really grateful, we
posted a shorter post in win2000 general by mistake, but any replies would be
fab.

we're going to be troubleshooting it tonight in about 5 hours from now, and
we'll be trying the bootlogging option.

Thanks again.

Rob.