You actually can connect your windows PC and your iMac by ethernet cord, it's pretty cool.
Here's what I did:
-Connect both PC's by ethernet cord
-On PC: Find what folders you want to share, and turn on sharing (little blue hand should come up)
-On Mac: I ran the SysStat widget on dashboard to find out the iMac's IP address
-On PC: Went to "Add Network Place" or something like that, and put in the IP address
-On Mac: The PC showed up under Finder-->Go-->Network, click on PC name and "Connect"
-On Mac: Log-in with username/pass that you use on the PC (if any) and your shared folders should appear
Hope that helps!