Your room. So lock the door, and keep him (and his friends) out. They’ve been making locks for doors for hundreds of years now, and they are cheaper & simpler than fiddling settings on your computer.
Yep, that was one of my options, but that would entail letting the cats know the new location of their litter box.
Thank you all for the great advice and great ideas. 
#1 set passwords on all accounts, including the safe mode administrator.
#2 enable parental controls on all accounts (on vista these are fairly powerful and informative)
#3 check logs later
if he is getting into something it will be logged
Make sure the admin password is hard something like firstname-first4ofssn-lastname-belovedrelativesdateofbirth in some order.
If he is using a live CD you will not have a virus issue.
Side note, it would have been cheaper to ship it to me for virus cleaning in CA then pay $299 to the WeakSquad
No, Vista does not have a default blank password. And neither does XP. Some manufacturers distribute Windows pre-installed with no password, but the full GUI installer will not let you do this.
Also, the trick with a Linux “Live” CD to change the password does not work with Vista (yet).
The fact that the installer prompts for a password does not mean a password must entered. It is entirely possible to install XP with a blank administrator password, and the GUI does not complain. I don’t know about Vista, though.
It does.
What does? Vista?
Yep. I just set a passwordless login for admin on my Vista laptop.
Out of curiosity, I just ran through an install in VirtualPC for Japanese Windows XP SP2 (the only XP I have handy). The installer does not let me leave the administration password blank.