Are you using a router or a direct connection with a crossover cable? If you're trying to connect directly to the PC from the Mac without a router you'll need to know the login and pass that was set up for the shared volume on the PC; it's not necessarily the same as the user login.
EDIT: I see you added another post before I finished mine. If you're using the PC to log into the Mac, you might need to try the user/pass combination you use to log into your Mac.