Here is a very quick way to find out about how long a particular Windows system has been running :
1. Open command prompt ( access it either by clicking the Windows orb > All Programs > Accessories > Command Prompt or by searching for command prompt in the “Search program and files” box that shows up on clicking the Windows orb. )
2. At the command prompt, type
net statistics server
The output of line “Statistics since..” shows the date and time regarding how long the Windows system has been running since last reboot.
[ For finding the uptime of Linux systems, check out this post. ]