1) If your client firewalls are of the kind that create rules of their own, turn 'em off.
2) If they prompt you for rules when activity occurs, have both clients in the same room if at all possible; if not, have them attended by helpers.
3) If they need manual config, do so before you start.
4) Ideally, have an identical account full-permission admin account on each client; if it has a password, use the same one.
5) Create a share. Use any folder that does not have a parent folder that is privacy-enabled. Go to to the other client and browse for that share via My Network Places. Use MNP to create a shortcut to it.
6) Make sure at this point that the other client is manned or configured as per 1 to 3.
7) If you use Pro, consider disabling Simple File Sharing, which can be done on Pro clients even in a mixed OS network environment.