Okay,
If I understand correctly, your backup exec server crashed which was also your domain controller. If this is correct, then there are a couple of issues, and please if someone knows more, please interject:
1. If you use an external tape drive, then you could install the BE software to another computer on the network, catalogue the tape and then restore back to the original computer, but you will need to install an OS on the original computer pus any software that was installed before.
2. If you were running the .DR (Disaster Recovery) then it might be possible to set up another computer, re-catalogue the tape, then push out the recovery files to the original unit.
Advice: I always set up a seperate server just for backups. BE is very stable and has been reliable, however any time you make a server pull multiple duties, you are running a risk. Domain controller and web server, okay, file server and lite apps server, okay, backup exec and anything else? Not so good. I hope this post has been helpful to you.