With Win 7 Pro, you can communicate with XP, (or Vista) without any special concessions. However, you can't form certain aspects of a home network with mixed OSes, they all have to be Win 7.
With Win 7 Pro, you are entitled to download a copy of "XP Mode", and Windows "Virtual PC". This is a virtual machine program, that for all intents and purposes, is a valid copy of Win XP Pro, 32 bit. And it's free!
So, you don't have to downgrade anything, if you have 7 Pro, you've already got the makings of XP Pro SP-3
All the info you need is on this M$ Download Center search page....
http://www.microsoft.com/downloads/en/default.aspx
I should add, the memory requirements to run the system in a virtual machine are high. I suppose that a 64 bit laptop would, or should come, with 4GB of RAM installed, but if not, opt for that amount of memory.
