I had this problem when i set up a network at home. Took me ages to fix it. Firstly ensure that your drives / directories are shared as required.
Secondly, from memory, i had to add the user onto the machine that cannot see the other machine. Your can do this by going to control panel, admin tools then add user i think.
It was a long time ago, so can't remember the exact details, but hopefully point you in the right direction.