all of your devices connected to TD-W8901G are in a home network and you can share your files and printer between them.
if you did not set firewall rules in your computers you can see the computer names connected to TD-W8901G in Windows 7 --> My Computer --> Network
it would be better to define static ip address for your devices.

i disabled firewall and i can see pcs in Windows 7 --> My Computer --> Network but i cant access it and i cant ping it

if you can see other pcs in Network and you cant ping them , it might be because of other side pc's firewall rules,
set the network location to HOME NETWORK, further more you have to use username and password to access the pcs, if they have not password you can disable password protected sharing but for security reasons its better to define password for your user accounts