There are a few possibilities.
What kind of sound card are you using? If you're using onboard sound it's quite possible that your recordings will sound like crap. Onboard sound is barely adequate for reproducing sound (what most people do: play MP3s, games, etc-- where non-audiophile quality is perfectly fine). Since not as many people record, it's really REALLY bad for that. You'll most likely need to get an additional sound card to really do good quality recordings.
That said, sometimes (for some reason) recordings STILL sound bad on the PC but great when burned to a CD and played back. Have you done any test burns of your recordings to see how they sound?
I'm not sure what you've been adjusting (other than input volume)-- I'd just make sure the recordings stay within a decent level but I wouldn't tweak anything except volume.
Here are my thoughts:
1. Get good cables (to go with your good sound card
). I don't mean $50 3' Monster RCA cables but some decent mid-priced cables.
2. Be sure you're recording at 44.1khz, 16-bit stereo. Besides being the CD standard, it is a fairly easy setting for a computer to record. A lot of software/hardware defaults to 48khz or some other setting for "improved quality" but it's all for naught since you have to resample down to 44.1khz for CD use, and unless you have a really nice audio editor with a GOOD resampling tool, it'll hurt the quality. (I haven't tried Audacity, I really should, I've heard good things)
3. If it's still not working to your satisfaction, you may need to mess with some more arcane stuff, like IRQ settings (if you get an external card), defragmenting the HD, etc. Real time recording is really sensitive to any issues with your PC-- anything that can cause the PC to "get distracted" (spyware, fragmented drive, etc) can cause glitches/hiccups in the recordings.
Hope this helps get you started..