This isn't uncommon IMO for cheap components. iv ownd some myself that exhibited crosstalk. Its usually not an issue though since the interference noise isn't noticable once you start using the audio you cant hear the interfience anymore. If you are still hearing it excessivly i'd guess its because your amp or some other stereo component is sensitive to the interferince or miss adjusted (sorry im nota stereo guy) but i do know just like software amps if your settings or gain is outa wack, or your just MAXing everything you will invite more noise interferience.
you know how it goes, you get what you pay for. The interferience is from within your computer, cause the cheap onboard audio controller doesnt have any shielding or suppression from normal interferience.
perhaps try the digital output instead of the analog? and see if you get lucky... or get an aftermarket Audio card.. as long as its not a cheap peace of garbage i'd imagine any would have better noise isolation then the one built into your board.
....or i guess you could just lower the volume a tad, your neughbors may appreciate it to