1. Make sure the workgroup(gong zuo zu) name of your 2 PCs are the same;
2. Username and password on the 2 PCs are the same; (make your life a little bit easier)
3. The 2 PC have different names;
4. "File and printer sharing for microsoft network" is installed(default setting)
5. Open MS Explorer(Liu Lan Qi), Tools->Folder Options->View, in the advanced settings(gao ji she zhi), UNcheck the last check box, "Use simple file sharing), click OK
6. Right click on any folder, select "Properties"(shu xing), you will see a tab named sharing(gong xiang), select it and enable the share. Set the permissions to whatever you want.
7. The share should be up and running.
8. Since you have a router the security concern should be minimal;
All above procedures only apply to the computers with default set up. It means you did not mess up with it yet.
Good luck.