Afrodite

Hi All I have recently installed Smart Security on a home network of 4 PCs all running Windows 7 and all connected by ethernet cable. The main PC can see and access all the other PCs, but the other PCs cannot neither see nor access the main PC. The system used to work okay and the only recent change has been the addition of Smart Security. I have been scratching my head over this for weeks now. Does anyone have any ideas where I am going wrong? Many thanks in advance for any help.