You could also just type in without the quotes "systeminfo" at a command prompt and it will tell you all sorts of things about your system, including Uptime.
System Up Time: 0 Days, 0 Hours, 30 Minutes, 34 Seconds
It even tells you when you installed the OS. Pretty cool info I'd say, since I cannot remember for the life of me when I last installed an OS... (Now I know - Feb 2004)...
-SlickVic78