In Windows 2000/XP go to task manager and see how long the system idle process has been on for. Since the computer spends 99% of its time idle this is a pretty good indication of how long it has been switched on for. I'm not sure about the logged on bit: if you are on a domain then the DC can audit the logon times and in Exchange 2000 you can view it through System Manager, Administrative Groups, Servers, %server%, %server% Store, Mailbox Store, Logons (since most people have Outlook opened all the time).