85% of XP users are reported to be sticking with it (and, as far as my desktop is concerned) I'm sticking with it for the time being.
Vista was always regarded as a bit of a disaster (although I've never had any real problems with computers using Vista) but it's nearing the end of its 'official' lifetime anyway. (Mainstream support for it ended in April 2012. Extended support will end in April2017)
Windows 7 is one of Microsoft's better offerings. It's similar enough to previous version of Windows not to have users tearing their hair out because they can't work out how to complete a simple task and also very stable. While mainstream support for it runs out next January , extended support [which is what actually matters to most people] continues until January 2020.
Windows 8 was loved by a few, but loathed by many, when it first came out. It seemed designed to work with touch-screen devices (so that Microsoft could do battle with Google's Android OS), leaving desktop users bewildered. There were also plenty of other problems with it too. Windows 8.1 seems to be a definite improvement but a lot of older software (and some older hardware, such as printers and other peripherals) won't run under it.
If I was buying a new computer now I'd definitely be looking for Windows 7, which means steering clear of places like PC World (where they only offer Windows 8.1) and searching out the independent traders who are still offering it.