you do not need a receiver.
if you wanted to use the whole system (speakers + sub) you can use a straight 3.5mm to 2x rca adapter likely. the rca are likely an aux input.
if you wanted to use just the subwoofer and it has an AC plug... its powered all you would need to do is adapt from rca to 3.5mm in either the same manner as above (if you wanted to run it with a pair of 2.0 pc speakers you already have) but with a splitter on the port or if part of a larger system (or with a center channel you already have.. we could adapt it to fire on the actual center 3.5mm jack as well.
if its an unpowered subwoofer (no ac cord going into it) you could either use the main box of the stereo set to power it (through an aux input jack) or use an...