okay. That means that there's something strange going on here. Okay, I just launched one of my WinBoxes. XP Home. Nothing shared. Let's see what happens when I try to set up a share point... Let's pick a random folder on a volume other than the C: drive. Set up network permissions, share the folder... Okay. I can connect to the share. I can view the share. I can open files, move files, write to files, delete files, create files. That's all with the username on the XP system set to be the same as the username on the Mac. (different passwords, though.) Let's try a different username. Generate a new user. Generate a new folder. Put some stuff in there. Step through all the permissions stuff... can I log into the share? Yes.
Okay, I got it to work first time on a vanilla XP Home, SP2, system. Took maybe 30 seconds, not counting time to boot the WinBox. All I can say is that either you have a configuration problem on either the WinBox or the Mac, or there's something Very Strange™ going on.