in order for the computers to see each other as well as been conected to the router they also need to have the same network name ie mshome, workgroup etc it can be anything you choose. obvoiusly you other computer have this same network name as they already see each other so it would make sence to find this name and then rename you new computer the same thus allowing it to join the network.
1. on 1 of the computers already part of your network > start > mycomputer (right click)> properties computer name . look under workgroup whatever name is there is you network name
2. repeat above on you new comp (it is a little differnet been vista) change the network name to be the same as the others then restart
all your computers should now see each other