Windows Logs

[Vitron]

Does anyone know of any way to tell when your computer was on? A pre-existing windows log, not a program. Thanks.

[Fallen Angel]

Does anyone know of any way to tell when your computer was on? A pre-existing windows log, not a program. Thanks.

Depends on which version of Windows you're running.
For all NT-based versions the system's securitylog should provide all the info you need. You'll have to manually select which events should be logged though (handled by the local security policy).
Using this method you can even log which user accessed a particular file and when (providing ofcourse, that you're using NTFS).