First, make sure that both pcs are set up with the same workgroup name but different computer names.
Secondly, make sure that the router has DHCP enabled and that both pcs are set up to receive an IP address automatically.
Thirdly, disable the firewalls on both pcs (don't worry, the router will protect them in the short time you will be like this).
Fourthly, make sure that file and printer sharing is enabled on both pcs AND THAT AT LEAST ONE DIRECTORY ON EACH IS SET TO SHARE.
You should now be able to ping each computer from the other, log into the router and see the other computer in My Network Places. You should also have internet access from both pcs.
If so, reset the firewalls and make sure that you allow the firewalls to access IP addresses in the range of your network addresses, eg, if the IP address of the router is 192.168.1.1 ensure that the range 192.168.1.1.to 192.168.1.3 is allowed on the firewalls.
Let us know how you get on.