Not sure why you want to see when your computer is booted...like Juridan said, just set a password on your account, set the BIOS up so you need a password to boot, lock the case - there you're mostly safe.

If you want to track when your computer is booted just turn on auditing of system events and logon and logoff. In Windows you do this via Local Security Policy and then check the security log for logon, logoff, shutdown, startup events.

Good luck.