|if you want to switch between outputting the sound from the computer to either your PC speakers or hi-fi components you could use something like this : - see the B-Tech 3 way control (2nd item down)|
You would use a mini-jack (3.5mm) to RCA plugs - red (or black) and white for audio only. (See http://en.wikipedia.org/wiki/RCA_co... from your computers soundcard into the switch box and then from the switch box into the aux input on your hifi gear and the correct cable from the switch to your PC speakers.
If however, you want to use both PC speakers and hi-fi at the same time you could buy the splitter cable. the only disadvantage is that controlling volume levels would be difficult. to get around this i would advise obtaining a basic small 2 channel mixer (try your local DJ equipment shop) that has 2xRCA inputs and 1xRCA output. This is another piece of equipment to clutter up your desk
From an audiophile point of view, i would not use both at the same time and get a switch.
Aplogies for the switch examples if you are not in the uk but you should be able to get something like that your local electronics shop.