Protecting shared files on XP SP2 home

Well, I know it *supposedly* can't be done with a password, but I've heard of
ways to do it and I was wondering if anyone could give me some ideas.
Basically, there's one computer I want to be able to access the files. It's
OS X Tiger, and will always have the same IP address. All other computers
When I mount a share, it asks for a password, but, like I believe all XP
systems are, any password will open the files. So I just need anything to
keep access to that one computer. I've heard about setting a password on the
guest account, but that doesn't seem possible. Also tried making a rule on
my router's firewall, but that didn't work. Is there any way to set up the
Windows Firewall for this? Any suggestions would be appreciated!
Thanks,
frosty