Mirosoft's Channel9 team has published an archive of all the Microsoft Professional Developers Conference videos dating back all the way to the keynote from the first in 1992.
Its easy to forget that only recently that streaming technology has made it possible for non-attendees to view keynotes live but of course, keynotes were not nearly as extravagant or hyped either. In fact in 1992 it's just Bill Gates talking for a good 45 minutes (with distractingly large glasses). Having said that, there are still a few gems, like Steve Job's appearance in 1996 and Jim Allchin's infamous Longhorn demo of 2003.
PDC 1996 Keynote with Bob Muglia and Steve Jobs:
PDC 2003 Keynote with Jim Alchin: