Show System Uptime in XP

1. Open the Start menu and select the run command,
2. Type cmd to bring up a command prompt
3. Type systeminfo

This will show a list of your hardware and OS information. If you scroll to the 11th or so line you will see the uptime.

Windows Vista changed this information to System Boot Date, but you can get the uptime in hours by loading the Task Manager and selecting the Performance tab.