Ok; so you want to share the PC's connection with the Mac via an ethernet cable? First of all make sure that the cable going from the PC to the Mac is a "Crossover" ethernet cable, not just a patch cable (Someone may correct me on this -- I'm not sure if you need a crossover, it depends on the computers' network interface)
To enable ICS (Internet Connection Sharing) in Windows, go to Control Panel > Network Connections > Right-click the wireless connection, select the Advanced tab, and check "Allow other users to connect through this computer's internet connection.". That should do it for the PC.
On the Mac, go to System Prefrences > Network. Click Configure on "Built-in Ethernet" and change the type to "Using DHCP" and leave everything else blank. Restart the PC and then the iMac.
If I remember correctly this should allow it to work.