According to Wikipedia:
Uptime is a measure of the time a Computer system has been "up" and running.in Windows you can follow one of the following methods :
The first one:
click start >> run and write cmd then click ok
write "Systeminfo" without quotes and you will find the system uptime
Update: there is better method suggested by Ahmed Barakat in the comments : instead of typing systeminfo and search for uptime in the results you can type ( systeminfo | find "System Up" ) to get only the system uptime thanks :)
The Second Method:
this is a great method if you dont like command line.
u can use an application to calculate uptime for u and there are alot of free great apps to do such thing one of them is Winlife 1.0.
it can be downloaded from here it is free and Small in size.
For linux users there is a command called uptime that u can use just write it in terminal and it will gives u something like this >>