I'm having the exact same problem and I am beginning to think it has something to do with routing tables but due to lack of knowledge can't get any further. I can ping the router from both machines but can't ping from one computer to the other. Just like you though the main one can see the other but not vise versa.
The setup must be ok as both machines can get on to the internet. The only difference I can see is I am using XP pro.