As you have windows 7 in both the desktops, you can go for homegroups, on any one desktop create a home group from control panel, and on the other system on the same network, join the home group created on first desktop.
You don't need to care about ips and all, it's very simple, and then you can share any folder or file by right clicking and selecting share in homegroup.
To access the shared files, go to network from search and there you would see the other desktops name, and you would get the shared files there, would work both way and like a charm.