99% of people who have 6ch (5.1) outputs on their PC's soundcard will have 3.5mm jack plugs for each channel (tho many will have stereo jacks that do both L+r fronts, l+r rears, c+sub) so you need either mono jack to rca leads, of if your jack outputs do two speakers each, then you need stereo jacks to twin RCA's.....

Then you put the RCA ends into the 6ch input on the back of your AV amp and presto, you have surround sound from your pc, nicely amplified by the AV amp.

Of course, if you already have the 6ch inputs on your AV amp in use, then you will have a problem....

Personally, I had a bog standard 2ch onboard soundcard and a pair of normal PC speakers. Didn't like em so for £15 I bought a Trust digital surround sound soundcard (main PCI card with the 5.1 outputs on it, then a daughter board pci card with optical and coaxial outputs and inputs) and connected my PC to my hifi using optical cable. Still only have 2ch stereo, but then I don't use my PC for surround sound playback (thats what the big ass TV and expensive DVD player, AV amp and Mission speakers are for!) and all I really use the PC for (sound wise) is MP3's, which are stereo anyway...... But it sounds fookin ace, and my hifi has decent amplification so it is loud and bassy