It's definitely rude. But I would password protect the computer from now on.
Did you look in the browser's history to see what sites he visited?
Did you look in the browser's history to see what sites he visited?
IMO, this is big time over-reacting. You haven't mentioned what he did on your PC, but I can only assume it wasn't anything bad (you would have mentioned what it was if it was something truly awful - porn, personal documents, etc).
From my perspective, he probably went upstairs and used a computer for a little research on something that came up in conversation, or to check mapquest for directions home - something harmless.
People get so damn caught up in 'this is my property, no touching, no looking', it's a really un-flattering and honestly, immature way to be. Yes, it was also immature for your brother in law in law (or whatever) to just use it without asking, but it's more of a minor annoyance than something to get worked up about.
Everyone just needs to relax and take things a little less seriously. You aren't protecting the world's deepest and darkest secrets on your personal vista box.
Def not trying to be rude, just my $.02