[RubberyDuck]

Does your main pc have a logon password set?
If not create a new user (maybe an admin just for testing, alternatively set permissions for this user on the share) give it a password, try mapping a share from the other computer but using the new user account (mainmachine\user) details.

You could also try disable passwords for accessing shares by doing the following:
Right Click Start > Select Control Panel > Select Network and Sharing Center (View by: Small Icons) > Select Change Advanced Sharing Settings > Scroll Down to All Networks and Expand > Select Turn Off Password Protected Sharing (Under Password Protected Sharing) > Click Save Changes