I have a dell xps m1710 laptop, works very well

I want to output the sound as either optical or digital out to my surround sound system, so can watch dvds etc off laptop.

What's the easiest way of doing this? is their a usb to ?? convertor I can use.

Thanks

 

Plenty of USB sound cards around, but I'd go for something more compact:
Behringer UCA202 2 In/2 Out USB/Audio Interface with Digital Output ::: £22.00 ::: Product Details at DJ Store

 

Or:

Edirol UA1EX USB Soundcard / Audio Capture Device ::: £59.00 ::: Product Details at DJ Store