how to connect speakers to my pc

So i recently moved. When we moved in. There were these speakers that are a surround sound set. I really want to hook them up to my computer, but I don't really know how to.
I know the speakers connect to a sub with copper cables. But I don't know how to connect to my pc.

Note - there's a volume control on the front of the sub. On the back of the sub there's two inputs. One says from sub out the other says from LFE out

Here's a pick of the back of the sub. http://oi43.tinypic.com/9vdylf.jpg

And just in case you didn't know. It needs to connect via 3.5mm audio jack.
So how can I do this.
Thanks in advance.

Most computers offer a Line Out. You connect powered speakers, or an amplifier and speakers. A few offer Speaker Out. An on-board amplifier supplies enough power to run a pair of small speakers. You need an amplifier. Old hi-fi units aren't hard to locate, and shouldn't cost you much.