Excactly. Time to get a new friend... Even if you somehow manage to prevent him from changing your password (which it seems is not possible) it is too great a risk sharing a computer in which you store importatant stuff with someone like him (after what he has done twice)...

Who owns the PC?

If it's your PC, just delete his account. Problem solved.

How come he doesn't need your password to change your password? Is he Admin? Ehm if not then you should be able to do the same to him...

Change his account to limited - there is no reason you need to have more than one admin on a home computer. A limited account can do anything a user needs to do on a regular basis. But cant change your password, cant format the drive, cant screw around. If you cant trust him with that much access, then dont bother giving him any and set the BIOS lock. The BIOS lock kicks in before the computer even runs POST, so there isnt really any way past it without the password. Secure the case though, if you can get at the mobo you can clear the lock in 30 seconds.

ok, we both own teh comp. and i can't change his account to a user's, hell delete everything. i suppose im just screwed.

As has been said a few times before if you change his account to a limited user he won't have the rights to delete anything not in his own folder IIRC.

Just a question, why does he have the power over you? If it is a join computer why would he do such a thing? Does he generally just not like you? Or is he just a prick who thinks it is funny to mess up someones work.

he can just reformat the C drive using the boot cd, and he does this when i piss him off. but he is REEEEALLY easy to piss off.

Me, I wouldn't mind sharing my car, my pet, my wife but I would never ever share a PC.
It was bad enough sharing it with my siblings, so I can imagine how it must be sharing it with someone who sees nothing wrong with changing your password... Luckily I have my own now...

Hmmm... I'd make sure to find that boot CD and store it where I knew it to be... and quickly.

Frankly, looking at the situation you're in... you apparently are sharing the computer with a psychopath. Consider checking the Sunday adverts for Office Depot, Office Max, and CompUSA and look for one of the entry-level computer specials. For less than $400, you can be rid of this problem forever.... and if you jointly own this computer, you can sell him your half and cut the cost even more.

Better that than the jail time you'll get when you beat the snot out of him after he decides to reformat the hard drive just after you've completed that 15-page final exam paper... and even though it would be justifiable, you'd still do time.