Educational, fun games for young children?

caimlas writes: My son is a lot like his father, in that he's addicted to technology — in his case, games. He's a moderately advanced 5-year-old, and spends every moment he can playing computer games like Counter-Strike 2D, Mechwarrior 3, (classic) Gameboy games like Zelda and Mario, and the like. He knows his alphabet and numbers, but he can't read yet. Additionally, my daughter is starting to show interest in such things. I am looking for a couple educational games to replace these purely entertaining games which will keep his interest. Something like, maybe, old Hugo Whodunit games. Basic requirements would be simplistic controls/interface, no requirement to be able to read, and an engaging game world. These games would be going on an older computer and would, preferably, be free or inexpensive.