Try a G4 running OS X, my fav. computer. OS X is very powerful, incredibly user friendly (hell , like it or not Mac OS rules user friendliness), yet you can get very technical with it if you desire, the Terminal is a unix style comand line terminal running straight off the Darwin kernel.
I can't stress the ease of use enugh on OS X. Considering that you have UNIX running the thing, OS X is probably the simplest, most intuative OS you can ever run. It keeps all the technical stuff just out of sight, but dosn't limit the user if he desires to toy with that side of the system. Because of this OS X is rediculously powerful and customizable, while retaining unparaleld ease of use.
OS X is disgustingly stable and has not crashed the whole time I have been running it. OS X can't be beat as far as user experience, security on it is incredibly tight, you have control of everything on your system, Apple does not spy on it's users and supports the open-source comunity by going open source themselves. The bundled software is fantastic, and the overal experience is the best around.
Or you can go with Linux on the PC (or PPC if your so inclined). Linux running one of the more popular desktops will work pretty similiar to windows (granted setup can be difficult, as well as adjusting to the new environment).
I have used it a few times on my friends computers, and while very diffrent from other OS's , it's merits are eveident. If your willing to put effort into it, Linux is an incredibly rewarding OS to use.
Check out the verious threads on the Linux forum. Or check some of the links on the front page.
Leaving M$ was the best descision you have ever made