If you buy a machine with Windows 7 Professional, the EULA agreement (or what ever they call it nowadays) allows you to install XP Pro legally.
If you machine is a tier one machine (Dell / HP etc) then you might have driver issues.  Research the motherboard type and see if XP drivers are available or not.  If they are, load up XP and be happy.

All the recent machines i bought for the office were ordered with W7 but with XP Pro pre-installed. Except one I forgot, staff member with it is the least computer literate and says she loves it, and everything works well. Might change mine and see what all the fuss is about. I did get one with W7 for one of my kids intending to have a play with it but forgot about that too - so the 8 year old is happy as well.

No idea what any of that amounts to !!