I am one of those Dads who allows my kids to go wild on my PC. If he is going to break something - no problem, being on the geeky side quickly rectifies any issues. A lot of parents may not be too happy with their child messing around with the computer, but with a little guidance and encouragement, you will be surprised at how much the younger ones can pick up and learn.
It is amazing what your kids can do when left to their own devices, I recently upgraded my main PC the the RC build of Windows 7. Apart from being a great update to the Windows family, the new version of the simple paint program is fantastic for teaching your kids basic computer manipulation skills.
ET has always had a big fascination with computers and gadgets of all sorts - it must run in the family. Here is what he came up with on his first attempt using "Paint" using multiple mouse buttons and advanced menu navigation: