Windows Server 2003:
In a command prompt, execute the following command:
systeminfo | find "Up Time"
Windows Server 2008 and 2012:
In a command prompt, execute the following command:
systeminfo | find "Boot"
Also, the command “net statistics server” will pull provide the server uptime, as well as many other related statistics.
This has also been included as a displayed statistic in the Task Manager of Server 2008.
Thank you,