Years ago there were child-oriented GUI overlays for Windows 3.1. Now there's nothing. Configuring a simplified or 'safe' version of XP is about impossible.
The Mac situation is much better: Configuring a simplified OS X machine: Simple Finder.
This is a good solution for younger children, elders (we'll all be there if we're lucky), and anyone overwhelmed by the staggering complexity of modern computer systems. Alas, there aren't a full suite of applications to match the Simple Finder -- maybe soon? (I need to play with Front Row and the Apple Remote a bit more to see what one can do with those.)