It there is a real grounding problem with the computer, you’ll notice that the hum changes with the fan and the display. This could indicate an internal problem (either a missing ground or maybe a motherboard problem). If this is the case the only way to get rid of the hum would be to have someone who really knows what they are doing to take a look at the PC.
It’s more likely that you just need to set up your equipment in such a way that EM/RFI is minimized. The following will all help:
- Make sure your audio cables are well shielded and not lying next to power cables.
- Connect all the audio equipment you are using to a single multitap (you may as well use a good one with a decent circuit breaker while you're at it), and then connect this to a single grounded outlet.
- Make sure you are using a well shielded, low emission monitor.
