One can't simply install 7 or 8 over XP and have all the previously installed programs "just work". They'd most likely have to be reinstalled on the new OS.
http://windows.micro...indows-7#T1=tab01
My step-mom just got an iMac and is in the process of transitioning over to it (with the help of a Win7 VM running on Virtual Box) because she didn't want to move from XP to Win8.x.
There apparently is some magic that VMWare and maybe Parallels can do to run a real Windows install. It would probably not be supported to keep XP for VMWare (or Parallels) and also install Win7 - you'd have to check. And some unsupported hackery apparently can be done to try to get VirtualBox to run a real Win install - https://www.virtualb...i/Migrate_Windows - but I'm not brave enough to try it.
Given all of the things that can go wrong with installing Windows under the best of circumstances, it probably will save time and aggravation to just back everything up and do a fresh install of 7. It's around $135 at Amazon (for the OEM version). The "Custom" install of 7 or 8 over XP apparently would still require a lot of manual intervention (to move stuff out of "Windows.old" to where it needs to be, so it might not save any time. See the MS linky above.
HTH a little. Good luck!
Cheers,
Scott.
(Who is considering moving from Win7Pro-32 to Win7Pro-64 and will have to do the same thing.)