Try the DAC in a different USB port. Some external ports on some computers are actually connected to a hub and share the bus with other internal USB devices (like keyboard, mouse, trackpad, camera, etc).
If that doesn't help then run the
DPC Latency Checker. If you get red bars in the latency checker you'll likely be getting audio glitches as well with a USB DAC (or even the onboard sound). The web page for the DPC latency checker explains how to hunt down what is causing the problem. Prime candidates are wireless network drivers. Disable devices till the glitches go away then look for updated drivers for the problem devices.
Microsoft did some work to decrease DPC latency problems in Windows 7. Technically, theoretically, Windows 7 should have fewer audio glitch problems than XP. But all depends on drivers and devices installed.