Thank you. I understand that there is no 3.5" jack for 5.1 surround sound. And, I don't actually have the DVD player, only the five speakers, including the goofy proprietary cable that looks like PS/2, only a bit bigger and more pins.
The reason I suggested an adapter is because there are six 3.5" jacks on the back of the computer, one for microphone input, one for line in, and the other four for various parts of the 5.1 sound channel. So, I would be looking for an adapter to take the four 3.5" jacks and rearrange signal to input to the sub.
Looking at the sound settings in control panel of Windows Vista, I have:
1) Realtek HDMI Output, which only delivers L R (and after I have written (3) below, it is probably part of that same card, because it says optical jack like below);
2) Speakers, which delivers 5.1 using four jacks;
3) Realtek Digital Output, which uses an optical jack and an RCA jack on my PCTV tuner cards (for which I could not find a Vista driver, so I am using the tuner card that came with the Media Center PC, which does not have these outputs).