For a mixed OP system network do not use the Home network location. Use the Work network location if you have an XP machine in your network.
Go through this tutorial about sharing with XP, click on the XP tab, it should help. Sharing files and printers with different versions of Windows - Help & How-to - Microsoft Windows
This one is more general but has good info. Networking home computers running different versions of Windows
After going through all of that if you are still having file access problems try doing this registry adjustment on the Win 7 machine with the problem.
Solution is really easy just modify one key in registry:
create or modify 32-bit DWORD: LocalAccountTokenFilterPolicy
set the value to: 1