There will be a bank of pins, with other little plugs there too, which will probably say something like 'hdd' 'reset' 'spk'. The 'SW' is the switch to turn on the pc.
The best way if it's not labeled on the m/board is to search the net for a diagram of your motherboard. Just get the motherboard number (printed on it somewhere) and put it in a search engine.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix desktop pcs.