The problem is that XP Home does not support domain login like Pro does. It sounds like your FW is using Windows integrated authentication. Map a drive over to your server in the same domain or FW (if you can) and have XP remember the username and PW. This will allow XP Home to have a set of credentials to try - your local login to the PC doesn't count.
If the thought I thought I thought had been the thought I thought, I wouldn't have thought so much.