Good old uptime doesn’t exist on windows boxes (you get to pick if I’m being literal or figurative), so in order to get the time since last reboot on a windows machine, fire up the command prompt and run this.
net stats srv
You’ll get a bunch of stuff back, but way at the top will be a line similar to
Statistics since 3/18/2008 3:42 AM
That’s the last time the server was rebooted.