What operating system are you using Mark?
It sounds like you are sending audio through whatever OS audio stack you have rather than through USB to converter.
There should be an option in your sound management to send only system sounds to the onboard speakers and all audio to a specified device.
K, you got XP.
Start here mate as if you were setting up a Dac.
http://www.ayre.com/usb-xp_setup.htm
Single spur balanced Mains. Self built music server with 3 seperate linear PSU, Intel i5, 16 GB RAM no hard drive (various Linux OS). Benchmark Dac2 HGC, single ended XLR interconnects/Belkin cable. Exposure 21RC Pre, Super 18 Power (recap & modified). Modded World Audio HD83 HP amp. Hand built Monitors with external crossovers , Volt 250 bass & ABR, Scanspeak 13M8621 Mid & Scanspeak D2905/9300 Hi. HD595 & Beyer 880 (600 ohm) cans.
The whole problem with the world is that fools and fanatics are always so certain of themselves, and wiser people so full of doubts.
-Bertrand Russel
John.