I have a large sub connected to my surround sound in the tv room. Can I connect it to my pc? It uses a rca plug but the pc only has the 3.5mm. If I get an adapter will it work? The sub plugs into an outlet for power.

3.5mm to RCA will work as the sub is active and has its own amp. However the quality will be poor, my suggestion would be to get a soundcard with RCA connections. I'd also suggest relying on more than just a low frequency speaker for sound, you'll never hear trebble or mid frequency sounds, just bass.

Of course I will need other speakers, I have them, but I need a sub for base.
So, how bad will the quality be if I use a 3.5 to rca?

Ok, I made the 3.5mm to rca cable and it works fine. But I cant set up the pc. I use the Klite codec pack.(4.10F). Im not getting the same as on the tv. Is there any program I need.
BTW I have 2 speakers and the sub, so I put the settings to 2.1.

How's it all set up? Are the three speakers connected to seperate inputs in the PC, or if the sub connected with the two speakers plugged into that?

OK - basics of sound now. Dolby digital or 5.1, 4.1, 3.1, 2.1, DTS etc are all different from stereo.

By the sounds of things your trying to play either an MP3 or a video with MP3 sound (this is common among divx encoded video). It seems unlikely that the format will be anything other than stereo. I've always had my PC feed to my amp and then just done the decoding through that though so what do I know. In really high quality divx AC3 sound is sometimes present and theres plenty of software scattered among the web to recode stereo to AC3 (I think VLC has this ability in its audio settings somewhere). Having said that I've never done anything like this - my PC is just a media server which I hook up to my amp which does all the encoding/decoding for me.

Also, don't ever make cables - especially things like RCA, RCA cables are extremely sensitive to EMI.

This ought to be pretty simple. You should be able to get a 3.5mm to RCA adapter from Radio Shack. You'll more than likely need a mono adapter.

Since the sub has it's own amp, just plug it into the 'sub-out' connection on your motherboard or soundcard, whichever you use, via the adapter. For the speakers to work (unless you're using a seperate amp) they must also have their own built-in amplification like your sub. Just plug them into the left and right front speaker outputs on your computer. The 'jukebox' (audio playback) in your computer will do all the converting, etc. from mp3, WMA, or any other compression you may be using to save space on your HDD. If the music is still on store bought CDs or was ripped in the original CDA format, no 'decoding' is required. Setting your computer output to 2.1, as you've already done, should provide the proper sound source.

BTW, I've made up my own 'RCA' cables for custom length runs and have found they are not sensitive to EMI (electro-magnetic interference). If they were, running component stereo (or home theater, etc.) interconnection cables would be a nightmare.