My Mp3 music was working allright
Where are the files located? Im guessing you ripped/downloaded them to your hd?
installed the USB card and the DSL modem
You lost me...what usb card? What about your DSL modem? Does it connect via USB?

Drivers, drivers, drivers! Windows Update cant be relied on when updating drivers, so try manually downloaded the newest ones off your audio-card vendor and installing them. Reboot.

Thats the only practical advice I can give, lack of details=lack of help

Good luck