OK first thing wrong is listening to anything from the Geek Squad. Sorry but I wouldn't trust those clowns (no offense to clowns) with changing a light bulb.
Also Nvidia isn't going to help as they didn't make the motherboard, all they made is the chipset. Asus made the motherboard and they won't help either as it is custom made for HP.
Next the SPDIF header on the motherboard is here,
now despite it being a 3 pin header, it only actually requires 2 wires. I am assuming the cable you have looks something like this one (minus the white connector), Attachment 91882
If so a sharp knife and a steady hand is all it will take to get it to fit. If the 4 pin connector is like the one above with both wires beside each other, then it is just a matter of cutting off the two unused ones, without cutting into where the wires are inserted.
Then it is juts a matter of connecting one end to the video card and the other to the SPDIF header on the motherboard. It may take a couple of tries to get it oriented the right way; it could be pins 1-2, 2-3, or at worst 1-3 which if that is the case will require splitting the now 2 wire connector into 2 1 wire connectors.