This problem is usually an issue with bus mastering on the motherboard, where the graphics device takes higher priority than the USB stream, causing the audio data to not be sent in time to the sound card. It's not due to any kind of noise from the PSU or anywhere.

I suggest playing with the apps jiiteepee suggested here. In particular, start the DPC latency checker and scroll around in firefox or MSN (or another slow graphics app that causes the glitches in the sound), and see if your latency goes above the red line.

I have the same problem with my desktop computer. You might also want to try going to your graphics properties, in the Troubleshoot tab, set the "Hardware acceleration" slider to "none".

Another option would be to change your graphics drivers to an older version or to a different version (NGO or Omega). If you're feeling more adventurous you could update your BIOS firmware, to see if that helps.

In the end these are just guesses because the problem ultimately is with poorly made hardware not playing nice with one another.

I know exactly what you are talking about. I had the same problem on both my desktop and notebook and it was a problem with thread priority. Not enough resources were being allocated to the audio stream. I use MediaMonkey / Vista/ Asio4all and all I did was adjust the thread priority in MM/tools/options/output plugins and set it to the highest and all that crackling stopped.

^ Brainsalad is correct, but there is more to it. The problem is caused by IRQ sharing and compounded by the fact that when the system is under load it handle the whole load without that noise. Its happened to me before, and I've seen it on friends systems etc. Its been a problem with windows for years.

My suggestion is possibly changing usb ports, or setting irq's manually. If you have an onboard soundcard, disable it while using the new one. Try this fix before any of the more extreme ones, as it fixes this problem nearly every time.

Oh yeah, forgot to mention that. Make sure to disable your onboard sound chip if you have one (almost certainly do). There should be a setting in your BIOS for that. Sadly there's no way to set IRQs manually (unless you want to reinstall Windows without advanced power management).

^ You can in the BIOS

Usually you can only select whether to allocate a particular IRQ to PCI or ISA (or whatever else is available for that m/b).