Windows XP was the last client version of Windows to include the Pinball game that had been part of Windows since Windows 95. There is apparently speculation that this was done for legal reasons.
No, that's not why.
One of the things I did in Windows XP was port several millions of lines of code from 32-bit to 64-bit Windows so that we could ship Windows XP 64-bit Edition. But one of the programs that ran into trouble was Pinball. The 64-bit version of Pinball had a pretty nasty bug where the ball would simply pass through other objects like a ghost. In particular, when you started the game, the ball would be delivered to the launcher, and then it would slowly fall towards the bottom of the screen, through the plunger, and out the bottom of the table.
Games tended to be really short.
Two of us tried to debug the program to figure out what was going on, but given that this was code written several years earlier by an outside company, and that nobody at Microsoft ever understood how the code worked (much less still understood it), and that most of the code was completely uncommented, we simply couldn't figure out why the collision detector was not working. Heck, we couldn't even find the collision detector!
We had several million lines of code still to port, so we couldn't afford to spend days studying the code trying to figure out what obscure floating point rounding error was causing collision detection to fail. We just made the executive decision right there to drop Pinball from the product.
If it makes you feel better, I am saddened by this as much as you are. I really enjoyed playing that game. It was the location of the one Windows XP feature I am most proud of.
Update: Hey everybody asking that the source code be released: The source code was licensed from another company. If you want the source code, you have to go ask them.