Did you re-iunstall all of your chipset/USB drivers for your motherboard? I assume you haven't changed any of the hardware unplugged or moved cables around?
There's filter drivers for some manufacturers that can help to reduce static in the USB ports, which may not attain to your circumstances as it's analog, but it still uses the USB ports for the internal DAC. However if it wasnt a problem before, it shouldn't be now.
The Tiamat is an analog headset, so it can pickup static/feedback from ungrounded components in your case. I've ran into the problem on another machine where there was alot of background static from the internal soundcard.
Do you have the latest drivers for your audio card
Have you gone into your windows sound config and selected 7.1 channel audio as your output?