The event manager in Windows XP allows you to find out when a system was rebooted or shut down. To run the Event Viewer follow these steps:
1. Click on Start -> Control Panel
2. Double click on the Administrative Tools
3. Click on System in the left pane. Resize the window so you can see the Event column in the right-hand pane.
Once you have the window open look for an entry of 6009 in the Event column. This shows when the PC was shut down or restarted. A start-up will be logged by a number of events, mostly with the code 7035, 7036 and 26, corresponding to various services and checks.
Source: Check a computer’s event history
– Suramya