Problem: Uni-directional file sharing

I have a DELL desktop and a DELL laptop, both connected to internet through a wireless router (ZyXEL P-330W) on Gateway mode. The desktop is on a LAN port, while the laptop is on wireless. I enabled file sharing on both computers, however, the sharing is only one directional. I have full access to the shared documents from the desktop to the laptop, but not vice versa.
1) When clicking "My Network Places", I can see shared folders of both computers on the desktop, but on the laptop I can't see shared folder of the desktop computer.
2) When I click "view workgroup computers", I can see both computers on both computers (sorry for the awkard description, but I think it is understandable). However, when I click the icon for the desktop on the laptop, it prompts a dialogue box asking for user name and password, I tried with the account that I use on the desktop, never succeeded (no error message prompts but keeps showing the dialogue box after I hit OK). Whereas accessing the laptop from the desktop never had such a harass.
3) Both computers can ping each other and get replies.
I have no idea what is wrong and seems other people had similar problems, unfortunately I am not able to find a finished thread to solve the problem.
Please help!