Look in the Event Viewer Log and in the System Log look for Events 6005
from eventlog which will tell you when the computer was started. It's
not an "official" way of doing it but at every reboot the Event log
service is started. When you shutdown the computer an Event 6006 from
the same source is recorded indicating that the Event log service was
stopped. The Event log service is one of the first and last thing that
starts and stops with the computer, and unless you would have manually
stopped and started the service it is an accurate gauge of reboot activity.
John