Sorry, don't have a solution ... but just to show you that you're not alone, I have an ASUS nForce 4 board too, installed Windows 7 64-bit, and like you, could not install my audio drivers
because it said there was no card connected.
I checked with the card vendor and they said (1) I had the latest drivers and (2) the drivers should work.
I removed 64-bit, installed 32-bit (and yes, I have a 64-bit, dual-core processor), and everything worked OK. Sound card detected, drivers loaded, audo config app works fine.
So, maybe there's a more general problem with ASUS nForce 4 boards, Windows 7 64-bit, and plugin sound cards.