when i use my webcam...or send an invitation, the other person doesnt see?

Please help me out here.When i send the invitation to someone,they cannot first of all see it..Icannot even see or hear them while they can see me.In Pakistan,i tried webcam with my dad,but he can see me but not hear me.I cant see him nor hear him.When i tried it here with a cousin of mine in cambridge,she can see and hear me but i cannot see or hear her.Please help.I tried everything.Reinstalling the web,windows messenger,everything.But nothing seems to work.I have win. XP and my web model # isREV:168z and it is USB PC camera..