Try this Win 7 Vista connection walkthough, click on the XP tab and follow the instructions. Sharing files and printers with different versions of Windows - Help & How-to - Microsoft 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.
If you are using an A/V software or a third party firewall it's usually best to uninstall them until after you have your network up and running.
If you are still having problems try this registry adjustment on the Win 7 machine which usually fixes this sort of problem.
create or modify 32-bit DWORD: LocalAccountTokenFilterPolicy
set the value to: 1