PC speakers have their own, small amplifier built-in — which is why they need a power supply — whereas your average home stereo speakers get their signal from the amplifier in the stereo itself. What you'll need to do is to connect the 'audio out' socket at the back of your computer to the 'aux(iliary) in' sockets at the back of your stereo unit — not directly to the speakers themselves.
The former is nearly always a standard 3.5mm stereo headphone socket; the latter will depend on what type of stereo system you have, but is probably a twin RCA jack. If so, there's a suitable cable for (item 12, p. 331 of the current Irish catalogue), but it's only a metre long. If you need more than that, try Maplins or Peats.