Audio troubleshooting - PC to Stereo Receiver

I am having trouble with audio from computer(s) to stereo(s).

I have an HP Pavillion running XP and using Realtek HD Audio device. I am trying to hook up to an Onkyo receiver. The cable, adapter and splitter (for another receiver, adcom) have gold ends and minimal connections, however, there is humming in all speakers (even when connected independently). There are three outputs on HP [rear headphone, front headphone and digital (I haven’t tried this connection yet)].

Interestingly (to me at least), when I hook up Sony Vaio (from headphone jack) there is no interference. Hook up IBM laptop (headphone jack again) – there IS interference. The difference between Sony and IBM/HP is that the Sony’s soundcard does not have digital or line-out option.

Basically, I will probably end up using digital coax from HP to Onkyo, but I really would like to understand why the above connection isn’t working and how it all has to do with line-in/out (or not). And more importantly, I want to split the connection and send it to Adcom system too (can I split digital and still receive quality sound if I do this?).

HP Pavillion
(Under Run System Info – this was all the info given)
Realtek hd audio
Driver v. 5.10.0.5449

(Under Device Manager)
There were no exclamation points or question marks.

 

Do I need to give additional information? Or am I in the wrong forum?