UpTime is a command line tool that will show how long your Windows installation has been running without reboot.

One can also press CTRL+ALT+DEL and then see when the user was logged on.
If logging off several times a day a more correct way is to run this command, which shows when the server service was started:

net statistics server | more

If using Windows Vista then this command will display the System Uptime (Or just open the Task Manager and see the Performance-tab):

cmd /k systeminfo | find "System Boot Time"