To find out when was the last time the Windows was rebooted:
Start Event viewer: start -> run -> eventvwr
On the 'View' tab click 'Filter..."
In the 'Event ID' type:
6005 to see when the Event Log service was started. It gives the message "The Event log service was started".
6006 to see when there was a clean shutdown. It gives the message "The Event log service was stopped".
6008 to see when there was a dirty shutdown. It gives the message "The previous system shutdown at time on date was unexpected".
6009 is logged during every boot.