The story says that the amount of download depends on whether users have the consumer version or the professional version.
Steve Ballmer told attendees at the London launch of Windows XP yesterday that the OS is "self healing", meaning that patches and the like are automatically sent to end users over the Internet.
20MB is just fine if you've got a DSL line but could be a bit awkward on dial ups.
You can find the complete story here. ยต