Help needed with PC/DAC

My Dell computer keeps rejecting my DAC (usb) and returning to its default factory sound outlet ("Realtek High Definition Audio"), which is definitely lower fidelity.
I have tried to find someone (here on Maui) competent to set up for me, but failed so far.
This has happened with 2 different Teac 301 DACs, so I suspect it is a computer issue.   The sounds are already downloaded on a big external hard drive (NOT livestreaming). 
Any advice appreciated.