Hey man. I've had success with XP.

I know you've tried all the solutions, and I'm going to outline EXACTLY what I did. BUT, before I do that, I want to mention something very important.

When you have the HD500 plugged in via USB, it becomes the sound card. Yes, we all know this. Say I have a particular application open, for example Firefox, and I'm watching Youtube while the HD500 is the sound card, if I suddenly unplug the HD500 so the default PC sound card is then selected, the audio won't be heard. That application, in this case being Firefox, needs to be restarted. It is because the application still wants to use the HD500 for some reason.

This is also the case when I have Audacity open. In case you don't know, Audacity is a basic music production/recording application. If I'm happily recording my guitar with the HD500 as the soundcard, and then unplug the HD500, I literally have to restart Audacity in order for the default sound card to start working.

I'm yet to find a solution to having to restart my applications all the time.

ANYWAY. To my solution to changing the sound card:

"Start" > "Control Panel" > "Sounds and Audio Devices" > "Audio" > under "Sound playback", in the drop down box below "Default device:", I select whatever is my default sound card, in my case "Realtek HD Audio output".

Be careful here. My PC lists 6 different devices that could become the sound card. If you're unsure, check this before you even plug in the HD500.

Bam, done. Solved. At least for me anyway.