Getochkn: Yes they're connected via the same power outlet. But they're on two different power bars. I have a surge breaker/protector on my pc and the bass speaker is hooked up to that one & I have another just plain multi-socket extension next to it.
I'm getting the feeling a DI unit might be in order ?
So the bass doesn't buzz and its hooked up the same power bar as the PC, but the guitar speaker and PC speakers buzz and they are not hooked up. Try plugging them into the power bar and see what happens.