You might have better luck asking this in a computer forum.

If it was me I would just seperate the computer stuff from the tv stuff.

Plug another amplifier (even amplified speakers) into the computer and see if you hear the hum from that.

If yes, it could be the mainboard's onboard sound processor. Most of these are fairly mediocre and shielding is piddle-poor. Shifting cables around or moving other electrically noisy parts (which is to say "just about everything") around inside the computer chassis sometimes helps.

Have you muted all the sound card inputs you're not using?

If all else fails, disable the internal sound processor in the BIOS setup and fit a good quality PCI soundcard. Turtle Beach cards used to be good but I don't know whether they are any more. Maybe Yamaha? It's probably overkill but Echo (echoaudio.com) makes very nice pro-quality sound cards. I can't recommend M-audio; I bought their Audiophile 2496 card a few years back and have regretted it ever since.

If the hum goes away with a different amp, you may just have a ground loop. This is typically a buzzing sound with overtone components rather than a pure 60 Hz hum.

The classic ground loop solution is to fit ground lifters ("cheater" 3-pin to 2-pin adapters) to one or more of the devices (computer, DVD, amplifier).

Also try adding extra ground connections among the chassis, and/or lifting the shield ground at one end of each audio cable.

Getting rid of ground loop hum is often a maddening process of trial and error. Good luck.