Is there an option in Windows that keeps track when your computer was last rebooted?
Just run cmd.exe with this command
For others solutions go to http://www.walkernews.net/2008/06/18/how-to-check-windows-last-boot-up-time/
It will be in the event logs.
There will be a lot of log entries in there, so click
You will then find entries saying such things as: