Sadly I share both of mine.

Kids get to use the laptop.  Only messed it up once, since they started randomly clicking stuff.  A real -long- talk about not leaving the site I load for them has ended that thankfully.  The daughter only uses it to check her email and the couple of groups on google she is part of.  The boys are the random clickers.  Needless to say they don't have the password to the desktop.

My desktop/rendering machine is currently being usurped for their dad playing Grand Fantasia (a free MMORPG) far too often when he is here.  And Poser 8 has fits rendering when he is on it.

No worries about him buggering it up, he is net paranoid, if it pops up anything at him he doesn't understand, I get yelled for.  Has been since I freaked at him back in 2003, when I spent a week removing porn popups from the comp we had then.  It was not pretty, or polite.