Determine OS on box

All,
Recently had a Windows server crash (this machine was used as a Terminal
Server). One of our technicians reinstalled Windows 2003 server on the C:
drive. Originally, the OS was installed on the D: drive. Now it appears that
the OS files are installed on the C: and D: drive.

I think the originally OS on this server was Windows Server 2000. I am
thinking that we downgraded the license to ensure that all of are remote
clients could connect without having to get a TS CAL.

So, my question to the group is this: Is there a way to look at the OS files
on drive D: to determine if Windows Server 2000 or 2003 was installed and
running prior to the recent crash?

Thank you,
Thomas Kroljic