airship, stick with windows 7, at least until you're sure you want to dive in to windows 8.
If there are people who absolutely cannot / will not migrate from XP, you might want to consider the mouthful that is Microsoft Windows Embedded POS-Ready 2009. Technically it's not XP but Windows Embedded as the name suggests... however, it's still NT 5.1 32-bit, seems to be 100% compatible, and you can configure it to look / feel the same (very easily, by copying Luna theme files from an XP PC).
What's more, it's supported with security updates until
2019, and that's not a typo.
More info (PDF):
http://go.microsoft.com/fwlink/?LinkId=159099