When you look at operating systems market it's quite obvious who dominates in this area. In all sorts of "windows" variants, Microsoft definitely leads. At the moment Microsoft is present on the market with two generations of OS, Vista and XP (XP can be categorize at this moment as "old school"). Both OSs are present in two basic variants: 32-bit and 64-bit. Vista also has several variants that don't have differences in performances, the only differences are additional options that can satisfy users' needs.