View System Up Time

To view the amount of time that your computer has been running with out rebooting or restarting the computer:

1) Open the Command Prompt (located under Accessories)
2) Type 'systeminfo' (without the dashes)
3) Once the system has finished gathering the information, scroll up and the time is displayed in the System Up Time/System Boot Time field.

As you can see, running systeminfo also gives a lot of useful information.