The only thing you can do if you're not able/willing to use password protection (BIOS, or login) is to do something physical, eg. pulling out the power cable to the PC & taking it with you. Removing the keyboard, something like that. I guess it depends on how PC smart he is, I think many PCs won't boot if they detect the keyboard is unplugged for example. Alternatively I guess you could do something like create another account, which is set as the default login (So you aren't prompted to login), configure the service in that login to disable some relevent services, e.g. Windows Audio - If it's Disabled, audio won't play. Should you want to use the PC you can then use the Start -> Logoff option & login to your account instead, which will have this enabled.