Some random thoughts on locking a share down a bit:

1) Make sure that tcp/445 and tcp/139 are blocked at your Internet/Home Network boundary (I'd hope they already are).

2) Use the built-in firewall provided with XP SP2 (make sure that you create a rule allowing the two systems to talk to each other).

3) Make sure the share is restricted ONLY to the specific directory needed (Ie, avoid sharing the whole drive or system critical areas (like \Windows, \Documents and Settings, etc). Make sure that the share requires authentication (preferably with a good password), ie no single dictionary words, upper/lower case, symbols, numbers, blah blah blah)...

4) Enforce LANMAN2 encryption/negotiation (much stronger and harder to crack)

5) Make sure you have auditing/logging enabled so you will have an idea if something has gone wrong (like a brute force attack). This will be especially important if you move to all wireless.

6) Make sure you keep your system(s) fully up to date in regards to patches/anti-virus (you are after all opening your system up by making the daemon accessible).

7) Consider using, if not already, NTFS so that you can further restrict the permissions of the share.