OK, I couldn't resist. Seems even a multi-billionaire head of one of the largest corporations on earth gets bitten by the demo effect. In this case, this means crashes of his Home Media Center edition PC and the blue screen of death from an Xbox during his keynote.
In my experience, most demo failures are due to lack of diligence on the part of the folks setting things up (which has pretty much always been your's truly, when I've done demos). The most likely cause here was that nobody actually tested the machines and software in question (the actual ones being used, not some other machines) during a dry-run rehearsal. Of course, using Mac OS X or Linux instead of Windows would probably have increased reliability, too.