Sorry for Bumping this thread (though its not that OLD ).
As told by Saroman, event viewer is the best tool for this. comes along default Windows Installation.
to be more precise. Open event viewer (Start-->Run-->eventvwr)
Proceed to application Category.
See for entries which has "WinLogon" as source ( there can be thousands of such entries if u dnt clear ur logs frequently).
Done , now u know all the instances when ur PC was booted up.