first of all you should probably use just the wireless router instead of having 2 routers, that will make things less complicated. Then make sure all the computers can access the internet.
once you do all that all you have to do is go to the computer that you want to share the folder to and then RIGHT click on the folder you want to share, then go to PROPERTIES, then the SHARE tabe, then click allow sharing, then from the same computer go to the START MENU-RUN- then type "cmd" without the quotes, then at the command prompt type "ipconfig" it will give you some numbers, just look for the one that says IP ADDRESS (it should start with 10 or 192 normally followed by other numbers) once you get that number right it down.
Now go to a different computer on the network and go to START-RUN- and then type "//x.x.x.x" supstitue x for the i[ number you wrote down.
example would be typing the following in RUN:
//192.168.0.2
after doing that you should see a window pop up with the folder you shared. Its that easy.