You were absent or away from your PC which was turned off. On your way back, you just notice that your PC is on! So, if you want to know the duration of how long your Windows XP system has been running, try this:
Click on Start
Click on Run
Type: CMD to open command
On Command type: systeminfo
Press the Enter key
Scroll down the list of information to the line that says System Up Time.
This will tell you in days, hours, minutes and seconds how long the system has been up.
This tip will also allow you to get information about your PC system specifications, hardware and software. Also you will be able to know exact date and time of when Windows Xp was installed, this applies for other versions of Windows!