Reemul's June 5, 2003, solution worked for me a couple evenings ago. A neighbor had installed Norton Personal Firewall 2003 on his XP (Home Edition) and ME PCs. I think this was coincidental.
Same symptoms: ME files accessible from XP but not the other way. The Norton Firewall looked correctly configured, but even disabling it on both PCs had no effect (and yes, the MS XP firewall was already disabled).
When I found this thread, I checked the other symptom Reemul mentioned ("Not enough server storage..."

on the XP, and sure enough that happened, too.
I changed the IRPStackSize in the registry from 11 to 0 on the XP, rebooted, and instantly the files could be accessed from the ME system.
Now I'm curious what the IRPStackSize is. My pure guess is that it's a server LAN networking limit for info that gets saved on a stack, and maybe installing the Firewall software nudged it just over that limit (even when disabled). Zero probably makes it unlimited. Again, just a guess.
I don't know if it'll help the problem pavewayammo's having, but it's easy enough to tweak the registry on the XP and give it a try. Remember to reboot after changing the value, too.