Neverwinter Nights can be picked up for $10 and runs on older machines. There is a huge amount of content out there, though the engine is a bit dated, people have made lots of things.
Minecraft is the mainstay in our household, perfect for a ten year-old. Build, explore, mine, and create your own world.
We had a good time with Trine, and up to three can play. It's a pretty game, though hooking up multiple mice and keyboards to a PC can be tricky.