Xbox 360 vga sound problem

Hi,

Been trying to figure out this problem for a couple days now and google consistently brought me to this site. Seems theres a lot of great people here who may be able to help me.

Here's the problem.

I have my xbox 360 hooked up to my pc using the vga cable with the sound running through the mini-jack thing. I also have a pc headset running through the green/pink jacks for playing games on the computer. My speakers are usb (logitech z-5). When I plug the sound cable for the 360 into the line-in jack it comes out of my pc headset instead of the speakers but I need the sound coming out of the usb speaker...hopefully without screwing up my current sound settings.

Any help is greatly appreciated.

Thanks,
Bryan