If your speakers are analog only (usually 3 mini-plugs, L/R, Center/Sub, rearL/rearR) then you won't be able to get surround sound without buying something. If your speakers have digital (S/PDIF) input (high-end logitechs come to mind, Klipsch promedias and Cambridge Soundworks are analog usually) then you can plug right in.
If you have analogs and still want to use them, you have two options. 1) buy a PCMCIA or external (USB or FireWire, preferrably the latter) soundcard. 2) buy a Digital-to-Analog converter (DAC). DAC would be the most versatile option, but would also probably be the least cost efficient, and I can't even think of any right now that are cheap enough to justify. Klipsch used to make one called the DD-5.1 that would be PERFECT, and, in fact, I want one really bad, but they are rare and fairly pricey.
Hope that helps!