"If Microsoft withdraw their support for an OS, as with say 98SE..." I don't know if it counts,but the 98se Updates still come from a MS site,at least they did just 48hrs ago(or has MS withdrawn support but left the old updates on their site?)
and for once I agree with DieSse when he says "how silly some aspects of copyright law have become".
American copyright duration depends on several things - whether the work was created by an individual working for himself/herself (life + 70 years), or by a team, working 'for hire' (95 years from publication, or 120 years from 'creation').
Usually, when you think of copyright duration, you think in terms of the author's lifespan, plus some bonus years. The American situation is the result of Congress repeatedly extending the bonus years in a desperate attempt to keep Micky Mouse out of the public domain. In October 1998, Congressman Sonny Bono (Yes, the singer who used to be married to Cher) was responsible for the copyright extension act which effectively stopped famous Disney characters moving into the public domain during the first decade of the 21st Century. Congressman Bono succeeding in saving the legal lives of Mickey, Pluto, Goofy, and Donald Duck for another twenty years, until 2023, 2025, 2027, and 2029, respectively, 95 years after they first appeared on film.
That's why Windows 98SE copyright has so long to go, in fact I was a few years out in my original comment - it only has 89 years left.