ah. you will want to get a stereo amplifier then.
if they are rather low power speakers you should be able to get one for about $15. you would just run either a 3.5mm to 3.5mm from your pc to amp or a 3.5mm to rca red+white from your pc to amp (depending on what kind of input the amplifier has).
look up your model of stereo online to see what the total output is in watts and estimate what you need for an amp (take the number and divide it in two ( ie if its 50w total power you would need an amp with 25w x 2 channels) estimated of course....
so yes you can salvage the speakers but you will probably need to use a little mini amp. the cheaper ones look like this. note: image is for giving you an idea what they look like only... you need to see what your speakers are rated for and pick one out accordingly. this particular model might not work for you depending on the size of your speakers.