I personally believe the biggest mistake that Microsoft made was not killing XP sooner. I had this discusion recently with a friend.
So I went from 3.11 to 95 then 3 years later I had 98. I then waited 2 years and I got ME (let's try and forget) which was such a huge mistake that XP was forced upon the world. So in 2001 I started using XP, not XP SP3 which some people are happily using now or XP64 SP2. Then the release schedule seemed to be extended, we had 6 years before Vista. I liked Vista, I had machines that could handle it. Then we waited a standard 2 years for 7. The release of 8 was 3 years later which is again a reasonable timeframe for updates.
So the problem is not that XP is awesome (it's not). It's that people got left alone with XP for 6 years. This would normally have seen 2 releases in this time, instead it saw none. This led to complacency in the market and has led to the problems now.
Remember, new is always better. They should have killed XP support in 2007 and have replaced it in 2004 in my opinion.