Hi, you're missing is to get al the machines pinging from one to the other. This basically means meansthey are on the same network. Only then can you share files.
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.
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\System
create or modify 32-bit DWORD: LocalAccountTokenFilterPolicy
set the value to: 1
Not insulting your intelligence, but have you tried checking the "advanced sharing settings" (you can get there by typing it in the orb search menu) and turning on network discovery, file sharing etc.
Try entering \\PC-NAME-HERE in your windows explorer on xp.