If your sound card has a pre-amp output,
and assuming your stereo has an 'aux' phono input you can use a phono lead to connect them.
Check on your sound card specs.
If you have on-board sound and only have a direct output from your PC to speakers then I know (in the case of car audio) theres a convertor available, that converts 'speaker wiring' to 'pre-amp' input, which would then connect to your stereo as above.
Your 'local' car audio or local 'maplins'click here would be able to advise in more detail.