Here you've got to choose among an unholy trifecta of MS-DOS 4, Windows Me and Windows Vista. Released in 1988, MS-DOS 4 was notoriously buggy, and many applications refused to run on it. Users commonly reverted to MS-DOS 3.3 or jumped ship to Digital Research's DR-DOS 3.41 to avoid MS-DOS 4, Microsoft's first serious misstep in operating systems. Windows Me, released in 2000, was buggy as well, and plagued by installation problems and a plethora of hardware and software incompatibilities.

But I give the "most reviled" crown to 2006's Windows Vista, which proved to be far more of a fiasco than Windows Me or MS-DOS 4. The five-year gap between the release of XP and Vista was the longest gap between versions of Windows ever, so people had high expectations for Vista. Unfortunately, it was bedeviled by hardware incompatibilities at launch, it wouldn't run on older hardware, and many people disliked its resource-hungry user interface.

Making matters worse, many PCs that were tagged as "Vista Capable" -- a situation that led to a class-action lawsuit against Microsoft.

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