Despite JackMDS's objection, I don't think this will be a problem. All you need is a 1/8" (AKA 3.5mm) stereo male to 1/8" stereo male cable long enough to stretch from the back of one computer to the other. They can be purchased at RadioShack for as little as
$3.95+tax, though I recommend getting a
shielded cable ($6.99+tax). Just plug one end into the guest computer's line-out, and the other end into the host computer's line-in. Then load Volume Control on the host and make sure the line-in channel is not muted and that its volume slider bar is at an appropriate level.
Keep in mind that for this configuration to work, the host computer must be booted to Windows in order for the guest computer's sound to be heard.
To set up a configuration whereby both computers can be connected to the single pair of speakers without relying on a booted host, you'd need some sort of a four-(or more)-channel mixer. These can be a bit expensive compared to the $5-$8 cost of the afore-described cable chain. Radioshack has (
a 4-channel mixer for $39.99+tax). Keep in mind that a mixer requires appropriate cabling, which you're unlikely to have handy. In the case of the above mixer, you'll need two 1/8" stereo male to two phono/RCA male adapters, available at Radioshack for
$4.95+tax/each or
$7.99+tax/each for shielded cables. You'll also need a third cable to run from the mixer to your speakers or pre-amp (what you need depends on your speaker setup).
All things considered, I recommend the chain method. Though the host must be booted for the guest to be heard, that small drawback is offset by the relative ease and low price of setup.