Do all computers output digital thru USB?

Forgive my newbee question but I want to output a digital signal to my USB DAC from my computer. How do I know if my computer outputs digital thru its USB port or analog? How do I get a digital stream to output to my DAC? Thanks for the look and the help. Really.
Go to
Control panel>>Sounds and Audio Device Properties
Click Audio Tab
Change Sound Playback default device to USB speakers

and then click apply.
Thanks Swampwalker for the speedy reply and the help.
Just tried it and still can't figure it out. I am using Windows Vista not XP so it seems to be different. can anyone with familiarity with Vista help? Thanks
The computer should set up the DAC as soon as you attach the USB cable. If not, the DAC probably comes with drivers that should be loaded first.
Many newest stuff, requires a previous installation of drivers in order to work properly. Like riffer said, maybe you need to install drivers first.
Sorry, Pugstub. I've got XP.
OK, thanks to all those that responded.
All USB devices are digital. USB is a digital standard. There is no way to force USB to output an analog signal. Devices such as USB speakers have a digital to analog converter built in.
on your PC you can also pull the digital audio off the PCI or PCIe bus, if you have the right card, and go to AES/EBU.
Your best option is probably SPDIF digital out to DAC. USB limits you to 16bit, 44Khz. With Spdif you can go24/94 or higher.

Another option is the $150 M2Tech Hiface gadget. Converts USB - SPDIF (RCA/BNC).
My computer has a SPDIF out and it sound really really bad.
I tried going from PC to a Theata DAC and then had a panic attack wondering just how much $$$$ I could spend to get it to sound right.