AC 6.5 can run on XP for clients but NOT the main computer.
This is because SQL2012 is NOT compatible with XP, and needs to be installed on the main computer.
I would agree with the attrition route. Certainly if they are running XP, you can move up fairly inexpensively compared with adding Win7 or Win8, both of which will cost you at least $75-100. Better than that is that most newer computers come with more RAM memory. RAM is your friend, always.
Another option is leave your Server 2003 and pick up Windows Home Server (it's still inexpensive on Amazon) which is Server 2008 based. Then only use it for serving AC and leave your old server for doing what it has been (except hosting AC.)
