MS really, really doesn’t want to flood the secondary market with WinXP DVDs, or they’ll kill any chance for Vista to make serious $$$. If they can force you to buy a new computer with the OS pre-loaded instead of buying a naked machine and re-installing from your XP media they make more money (and hurt the chances of alternative OSes)
Microsoft is making the most basic mistake of business. They're doing what's best for them, not what's best for their customers. People know this, and they feel used. It's bad for your customers to feel used. The worse MS treats its customers, the more pressure there is to find an alternative.
If MS had actually listened to customers and made their experiences the best possible, customer loyalty would be so high that there would be little interest in Linux. Instead, we are made to get by with thinly-veiled fodder for future MS revenues, masquerading as useful software.