Chris Pirillo has a really good point about the new Leopard release. Apple trusts you. They don't require activation. If you buy the one disc version, it trusts you to install it on one computer. If you buy the family pack, you can install it on up to five computers. The problem is that Microsoft doesn't trust you. It has a complicated authentication process and digital rights that returns a large number of errors that disables Windows for users who have a legal copy.
Check this out.