Good things come in small packages...
Take a look at an NT command, "net statistics", which apart from providing all the vital network stats also shows the startup time of an NT server or workstation.
The syntax of the command is:
"net statistics server |more" for server side stats
"net statistics workstation |more" for workstation side stats
The "|more" part is important here as the start time is displayed at the top of the command output.
Startup time of a server can also be seen in the WINS Manager, of course if WINS service is installed. But as one can imagine it shows the start time of the WINS service which is about the same as the startup time of the NT server itself. Since WINS service is set to start automatically it's start time can be considered as the start time of the NT server. But for this you have to be 100% sure that WINS service wasn't stopped/started along the way.
Event log is also an acurate way of doing it but can be little tedious for servers running for months.
There're some other ways too, but of all, I think "net statistics" command is the best.
Hope this helps.
Mubashir