I don't have a USB soundcard, but I do have a USB mixer... the tascam us428.
i was having some problems with pops, clicks, and stops until I downloaded service pack 1 of the WinXP operating system.
now everything is rock solid. i'm pushing out 16 tracks of audio with the necessary directx stuff turned off, and am able to record a stereo signal at the same time... on a laptop, with a single 5200rpm hard drive.
if i turn the DirectX dynamics processing back on, I can't record any additional tracks with the 16 tracks pumping, but I can playback all of the tracks with the Dx effects on when I'm mixing.