If you have a separate modem and a Belkin router then presumably you have *CABLE* broadband from NTL/Telewest?
That being the case, the router's WAN port remains connected to the Cable Modem's ethernet port and both computers connect directly to the router, either by ethernet cable or wirelessly.
In each PC, set any folders that you want to have available over the network as "shared" by right clicking and selecting the Sharing & Security option. At least one folder should be set as "shared" in each PC.
Configure any firewall software to allow access to the networked computers, which may involve entering their IP addresses in a "trusted" section.
That should basically be it. Go into "My Network Places" and expanding the folder tree should show *BOTH* computers in each PC. Expanding further, should show the "shared" folders.
As regards the external drive that, or folders within it, can also be set as "shared" and available over the network.