Did you put the drive letter after the last backslash? If not, that's the problem, because it found the computer, but didn't know what to share after that. Say your XP computer has an IP address of 192.168.1.5, and you want the C drive, you type \\192.168.1.5\c or maybe it's \\192.168.1.5\c$
I know the dollar sign works on NT machines and 2000 machines, but I don't know about XP machines.