I know we've got a lot of home theater buffs here... Was hoping for some advice...

I have a BST 4 channel DJ mixer (I used to DJ in college) which allows me to mix in my PC, VCR, Empeg, and CD player outputs. Obviously I rarely need to play two sources at a time, but it's great to have audible alerts from the PC over my stereo speakers while I'm listening to music or watching TV.

Anyway, with my new apartment has come a new problem with my setup: noise. I don't know why, but I seem to get a lot of noise on both my audio and video signals in this apartment. I have some pretty long RCA cable runs, and in general my cables are basic Radio Shack jobs, none of those gold plated super cables or anything. Do those really make a difference? Are there shielded RCA cables which are less succeptable to this? The noise is a humming noise that sounds like a guitar amp that's turned way up.

My CD player and Empeg are short cable runs, and they're fine. However, my PC and my VCR in the living room are longer, and seem to have a lot louder of a humming noise. My PC's sound card is a SB Live Platinum, and when I plug headphones in, there's no noise to speak of. Once I run it into my mixer or my stereo inputs, I get noise.

I guess I'm wondering if better cables can fix these problems. I've tried swapping various cables (the ones that will reach anyway) with no luck. Are there other things that could be causing this problem? I've made sure my 4-channel mixer gain is set right, and I've turned off all of my PC's microphone and aux inputs to avoid any problems coming from those. Are there any other typical sources of this humming noise?

Thanks for any and all help, as usual.