I struggled with this as well
I have two desktops, one Win 7 and one XP, and a laptop with Win 7 on it.
All connected via a router and in the end I had to go back to the beginning and create new accounts on each using the same username and password on each.
eg: If the Win 7 desktop already had user MarkFlax on it with password 12345678, then I created new accounts on the XP and on the Win 7 laptop using the same MarkFlax username and same 12345678 password.
Only then did I start trying to share folders. But I found out that Win 7 accounts are restricted to Public folders, that is, accounts on Win 7 cannot see folders for other accounts on Win 7. In fact, you don't even know there are other accounts other than looking at User Accounts in the Control Panel. And that is with full Admin accounts.
With that in mind, from an XP system or another Win 7 system I can share contents of Public folders, but not much else.
However, sharing XP folders seems easier as security in XP is not as tight.