Open your firewalls and make sure that in Exceptions File and printer sharing is enabled. Make sure both computers have the same network name i.e. Workgroup. If one of the computers is XP Pro - enable simple file sharing. Put a test document into each computer`s shared folder. Ensure that each user login has a password. Reboot both computers after these procedures.
That`s a quick list of things I`ve had to do in the past!