Option 1:
 
    1.  Go to "Start" -> "Run".
 
    2.  Write "CMD" and press on "Enter" key.
 
    3.  Write the command "net statistics server" and press on "Enter" key.
 
    4.  The line that start with "Statistics since …" provides the time that the server was up from.
 

•   The command "net stats srv" can be use instead.

 
 
Option 2:
 
Uptime.exe Tool Allows You to Estimate Server Availability with Windows NT 4.0 SP4 or Higher
 
http://support.microsoft.com/kb/232243