I am the same way, after shareing my comptuer for my freshman year at college with my stupid roommate, I cannont share a computer ever again. He would fill up my HD with pointless crap, install everthing he could think of, change all my settings...
No i lock my puter whenever i am out of the room, but my roommates now now better than to come near my computer.
I absolutley hate it when people touch my monitor, i yell at my boss everyday for it...