windows 7

Yes buy Windows XP.

Windows 7 is a new operating system and not ment to be a clone of Windows XP, to be very honest XP is old and starting to show its age, Windows 7 is far more advanced and may just take a while for the die hard XP users to come to terms with but you will have to deal with OSes like Win7 at some point as WinXP will be lossing its security updates soon, so your out on your own with any new exploit that may come along (may not be Windows Xp but maybe a 3rd party run in XP as the cause of the hole)

But what you mean by make it act like XP? is it the look, the menus etc ... you can make it sort of look like XP but its backend is much mre advanced and the memory management is far superiour as is crash protection (have to add on crash protection and reliability is the caveat of "IF" (added in brackets as a generalisation and not aimed at you or anyone) you run warez apps then stability in any OS will be impaired.

Yes you can install XP, but this is reliant on your hardware having XP drivers, so check that XP drivers are available and if they are then you will be ok.