ALL of our customers are sticking to XP, some are still 98 but our latest software runs on XP and 2000. The same OSes run our DOS software fine, with no real issues apart from the clock going into sleep when a DOS programme is in background.
In our office mainly XP, 1 2000, 2 98 - I have 98 and XP under my desk.
One customer tested Vista but found XP preferable to use and they have their own IT department, they tried our latest software and we were blocked on registry, the other company they used (on one site didn't work at all).
Vista is basically an answer without a question.
As to software programmes do not get written overnight, our just ending life cycle software lasted for 14 years, the windows software has taken 3 years so far and is still being added to. There are loads of vertical market software solutions of a great age, this is how it goes.
Of course we could all outsource to programmer farms in India - but then what is the point of being in business.
What is interesting is that it is only the PC market which forces frequent rewrites, whereas not much changes on minis and mainframes.
Trouble with XP, is its old. 7yrs old. Way past retirement. And written in a bad era for MS, where market pressures force them to release prematurely. The primary reason Vista was late was to fix XP as best they could (XP2 is a major, major update, and hence should not be installed on a used system, but put on clean). In an effort to make it more Win9x (supposed to be merged with W2K, but couldn't get it working in time), its severely kludged, and suffers from thread races.
If your software is getting stopped at registry, you have been poorly programming - you need to brush up on the right places to store data

. All but one of my apps I've written has not issues, as I follow the rules (the one that doesn't follow the rules, I modify registry permissions to that part of registry).
Nobody, but nobody should be using anything earlier than Windows XP in any environment now.
Minis aren't really that popular any more - Vax was probably the last of the true minis, and don't exist anymore. I guess AS400 might almost be considered a mini :-/. Things tend to stay more stable in the server market, though there have been changes that can break things in most OSes. Novell is the only really big shifter, going from the horribly fludged NetWare core to Suse.
As for other desktops, Windows is relatively stable. Macs have gone through massive changes, from 68000 to PowerPC to i386/x64, and from Mac OS to OSX, each requiring big rewrites (the emulators did a reasonable job though)...