Howie;
This stuff can get pretty boring and a bit confusing so feel free to scratch your head and say "What!!??" a few times.
The username and password that is being used on the Windows XP machine. That same username and password needs to be on the Windows 7 machine (if not, add a new user) (you don't need to actually use the account it just has to be there with the appropriate permissions). Then that's the user you'll want to add (edit) to the access list (Group or user names) under the security tab for the folder that contains the file you are trying to open from the XP Machine.
Access to shares are controlled by both the sharing tab, (share permissions are for control of users comming at you across the network), and the serurity tab, (ntfs permissions) are for locally logged on user, however they are automatically combined when you create a share on an ntfs volume and when combined the most restrictive is always applied. That's why I recommend setting share permissions to Everyone = Full Control and then using the security tab to adjust NTFS permissions explicitly and with more granular detail. This used to be Microsoft Best Practice, but I think they've changed it, but I still do it that way, it just seems a bit easier and a little less confusing to me.
I might also recommend that you practice on a "Test Folder" just make a new folder add a text file to it and practice sharing it to your XP user, this accomplishes two things number one it's a safe way to practice with more nuanced security settings, (like if you find yourself having to break inheritance from a parent container and forget to copy (I think the call it "add" now) the already existing permissions you may find yourself locked out or the folder, this way no harm done, just blow it away and start over, likewise finding all the little check boxes under all the various advanced buttons to cascade permissions down through subcontainers and files can get confusing so this way you can relax and be a little more fearless in changing these settings. It will also give you a sense of having a template for setting up the actual folder that you're concerned with correctly, in that if your XP user can access your New Folder and open and edit and save the text file contained there in then it's likely that you'll be more confident that you have everything setup correctly in the real folder and it's not likely a permission issue generated by the OS if you continue to have problems but more likely a MYOB issue that needs to be tweaked.
Good luck and keep us posted
Regards
Randy