According to this article, Windows Vista is the main upgrade that's really a downgrade.
Rafe Needleman said:Vista
The obvious number one product for this list. Vista is the new shiny operating system Microsoft released to replace Windows XP. Except it hasn't, because it's a poor upgrade. It's slower, bigger, and buggier. Many people, not just those in the opportunistic Apple ads (and Apple has its own problems), would rather get a new computer with the old XP operating system.
Why it happened: Books will be written about Vista's failures, which, in fairness, probably have as much to do with Microsoft's need to support a vast universe of third-party hardware and software products as with flaws in Microsoft's marketing and software development strategy.