What you should do is buy a 3.5mm jack & red/white RCA adapter at radioshack/target/walmart and run the 3.5mm line to your laptop to the adapter and the red/white RCA audio cable from the adapter to your stereo (assuming you have free red/white RCA inputs available).
My stereo is about 10 years old but still works so I never felt the need
to replace it. It is a Phillips FW-C3 Hi-Fi system. I found a simple
3.5mm Aux Auxiliary Cord lying around so I thought I'd try to connect it
to my laptop and stereo. The laptop is using windows 7. When I plug
the aux cord all the way in on both the laptop and the stereo headphone
slots it doesn't work. However, if I plug it in halfway on both the
laptop and the stereo headphone slots, it will play on one speaker. I
know nothing about this kind of stuff so I need to know if I'm using the
wrong kind of cord or something else I'm doing is wrong.