The few times I have let family go on my PC, they have royally ****ed it up.
Last time it was my brother, going onto google and typing in something like "free mp3 download programme", and then downloaded the first programme that claimed to allow free MP3 downloads etc. This programme being some non-descript random thing that no one ever heard of and, of course, doesn't facilitate the downloading of MP3s but instead facilitates the skull****ing of your Windows installation with enough spyware and virii/trojans/worms to sink the Moon.
My brother didn't understand why I was annoyed, especially having lost over 6 years of university stuff (most of it was backed up, but that's not the point) and losing a lot of personal stuff that I didn't have backed up. Naturally I'm branded a "child" for getting annoyed at such things.
That annoys me the most about letting friends/family use your stuff - be it a computer or whatever - is that if you say no, they brand you as selfish. They can't see the irony that expecting you to give up your computer for them to use for their own gains is, in itself, selfish? That they don't understand the concept that it is *MY* property, and as such I don't want anyone using it, especially when they can't show respect for my stuff.
Thankfully, I live on my own AND don't let people use my stuff unless I a) have given them prior permission; and b) can trust them. Solves a lot of arguments.