Just a matter of knowledge and money

I would say that there are three major reasons why some people (regular folks) still use Windows XP (and I want to refer only to those who are very vocal against Vista):
1) They lack good IT skills and the knowledge necessary to understand why Vista and Windows 7 are SUPERIOR to XP, hands down.
2) They don't have the right hardware and/or don't know how to configure it correctly (see reason 1, above), and
3) They're cheap (cheapskates). As simple as that.
Companies, on the other hand, don't upgrade for the same reasons above, and the guys from their IT department are usually to blame (reasons 1 and 2). They usually are too comfortable using their old stuff (hardware and software) and too lazy to learn, so they advise companies not to use, for instance, Vista, because "it's buggy and slow, etc." Basically something they read or heard somewhere and never bother(ed) to try in the first place. I have withnessed companies using XP on machines with the Vista logo. How clumsy! they bought new equipment and bought also all the nonsense circulating about Vista and downgraded to XP (!), and on top of that they're still using Internet Explorer 6 (!). Again, if you still want to use Windows XP that is (literally) your problem, but what I find plain silly is getting a brand new computer (for those who can afford it) designed for Windows Vista and Windows 7, and then reformatting the hard drive to install XP (!) and then start comparing apples with pears...