Win XP, Win 7
Obviously no one seems interested in Vista (a good thing). And Win 8 doesn't seem to have much of a chance in most businesses except for users who are truly computer illiterate and can only function with touch screens (think cash registers at McD's). XP still works pretty well, especially on old computers, and lots of people are afraid of upgrading and its expense and training issues.
An area where this SHOULD change; Electronic Health Records. There is a huge Federal push for doctors and hospitals and the like to migrate from paper records to electronic. And privacy laws (HIPAA) mandate very tight security, and XP really isn't all that strong security wise. Also, the older PC's in many medical offices are not robust enough to move from handling bills and appointments to full blown medical record systems. So such offices will tend to need new PCs which brings the chance to more easily migrate to Win 7 while also upgrading networks and firewalls.
