The Logitech Squeezebox is a good option. The Squeezebox offers good sound and you don't even need the computer to listen to internet radio or music services such as Pandora, Rhapsody, etc.
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Since you said you didn't want to spend hundreds (e.g. no Squeezebox), a simple solution is a USB-Optical converter box like the Behringer UCA-202. They cost generally between $40-80. You can also get a modified one from the Tube Audio Designs website.
Another option is to get a cheap soundcard and use the digital-out to a good DAC. I wouldn't recommend using the analog out of a soundcard, unless you go for a pro unit like the RME Fireface.
All computers have an analog output. It's the green hole on a PC with a little headphone symbbol. You just need a cable that has a stereo 1/8 inch plug one end and RCA plugs on the other. Plug the 1/8 to the computer audio out and the RCAs to the preamp.Get the cable at Radio Shack, Best Buy, etc.
Not the ultimate in sound quality but the only conditions you stated were simple and cheap and this is the simplest and the cheapest.