So you want to be able to use the 2.1 speaker system that came with your PC in 2.1 mode and then switch over to a pair of bookshelf speakers in stereo mode connected to a seperate amp. Is that correct?
If so, then the easiest way I can suggest (I've done this) is to use a 3.5mm STEREO to Left and Right RCA cable to connect to your amplifier's inputs. This is a very common cable, with male ends. When you want to use the seperate amp with the 2 speakers in stereo mode, simply plug it into your PC headphone/ earphone jack. Typically, the PC sound card will detect the connection and automatically switch from the PC's 2.1 (SUB/SAT) setup over to the 2 channel stereo signal at the headphone/ earphone jack. Just remember to turn down...