It's supposed to work only in idle time, but what it thinks of as idle time (when the hard disk isn't working) usually doesn't jibe with how you see idle time (when you're not using the PC at all).
Plus, users don't have to worry about toting around a floppy disk or flash drive--all they have to do is log on to pick up working on projects where they left off.
Microsoft began working on the patches when its labs noticed the problem in 933 MHz and faster prototype systems that use ATA100 hard disk drives with a large physical cache.