how to lock out annoying guests

friend here needs some sort of a program to lock out his roomate from his computer. he logs on somehow and then downloads any and all kind of porn and viri. any suggestions on a good lockout program?

What's with all these crazy ideas? Just make a user account and a password, and disable Guest accounts. Not that hard. Set a password to monitor shutoff/screensaver and it will automatically require login if the computer is inactive for more than x number of minutes.

You could probably bypass this by booting safemode or using a live CD to write to NTFS- but who the fuck would do that?