I was reading some microphone tips, and advice they gave for those who "pop" when they talk, they recommend a guard made out of wire & an old nylon - or something similar will work. I don't know if that will help at all with your hum, though, that might just be feedback you can't help.
Also recommended is putting your microphone on something firm but not too firm, such as some cardboard, because a hum from a computer can resonate through hard surfaces. Maybe some paperback books?
I got a new microphone today - it's a USB headset, and sounds much better. Unfortunately, it's allergy season and my new microphone magnifies my nasality at least four-fold.
Three tries