One reason that you can't get your sound card drivers
to install is the completely unsupported NF3 chipset on the motherboard. Same thing happens with most AGP video cards on motherboards with that same NF3 chipset due to the lack of a proper and functional AGP/GART driver
(not to be confused with the actual video card driver). Same thing applies to your sound card and the lack of a proper chipset driver.
In this case without proper working chipset drivers, the operating system doesn't know what the motherboard/chipset is capable of. The NF3 was declared legacy of "end of life" back in late 2005, and as a result there is not now and never will be any Windows 7 drivers. Vista from what I can see came with a basic set of NF3 chipset drivers that at least allowed things to function, and unfortunately Windows 7 didn't.
This thread on the Nvidia forums has about the best explanation as to why with regards to AGP video cards, NForce3 250 + Windows 7 + HD3850 = BIG Problem - NVIDIA Forums
Have a good look over here, View forum - nForce Drivers • nForcersHQ.com
and see what you can find from others with the same chipset and how they got it to work with Windows 7.