The 3.5 mm male plug on the computer end,
a 3.5 mm male plug to 3.5mm (female input) Y cord splitter, terminating into two 3.5 mm male plugs. (A "Y" CORD)
one of these male plugs goes into a 3.5 mm female to RCA adapter. The two male RCA plugs will feed your subwoofer.
The other male plug will plug into a 3.5mm female to female adapter, and into the female end, the computer speakers plug in, like usual.
all you are doing is using a Y cord, and adapters to split the existing line level out, from the computer.
These type of adapters can be purchased at radio shack, walmart, or on the internet...