Make sure the windows networking part is installed
(client for windows networking) To check that, right mouse on Network places, choose properties, then click on your network. Make sure both PCs have this: If not, add it. But I'd be surprised if they didn't.
If you click on my network places and see the other PC, you're ready
Otherwise, you might right-click on my computer and choose network identification, then properties and check your name. Make both PCs the same workgroup name.
Then, right mouse on any drive or directory and look for sharing. Choose to Share this folder, give it a name, and you should be able to reach it from the other PC. If you're using passwords, you may have to add a username and password for the other PC. You can do this last step on both PCs and as many times as you want.