In the first place, connect the ULTRA LINK III to the pc. Turn it on... then boot up the PC.
XP 'should' see the new device and provide a driver for it.
PS Audio may and should be able to provide a driver especially for the Link III. At least they should be able to reccomend one that they know works well.
After the above...
OPEN THE CONTROL PANEL
Select "Sounds & Audio devices"
Select the AUDIO tab
In the drop down menus, select the PS Audio device for playback.
That should do it.
IF not you'll have to go into the Device manager find and then set the 'new' USB device as the default.
Mind that you continue to keep the Link III connected and powered up, when the PC is to be used with it. otherwise the PC may just revert to it's previous settings... or not. PC's are weird. I've two of them... one does what I tell it, the other does what it wants... sometimes.
Do let us know how or what you had to do to get things going... I'm interested in either the Link III, BC DAC III, and the Wavelength Brick for use with my main PC.