Re hiss, are you trying to feed a guitar pickup directly into a PC line-level input, and then cranking up the gain to get something out of it? If so, there's your trouble - as mentioned before, too little signal for the input. Get a preamp, or use a regular guitar amp and feed the line-out from there into the PC.

Recording to PC can be inherently noisy with some onboard soundcards. Some are pretty good, but some cheaper ones are crap. Also consider that the noise might not be on the recording. To rule out the recorder (PC or whatever) as the source of problems, buy or borrow a basic MP3 player that'll record and has a line-in socket. If that's noise-free, you've cracked it.

I had (still have, but it's awaiting Ebaying) an ST Audio 8-track PC recorder which had serious hiss on the headphone output. Try playing back on some other medium, like burning to CD and playing on your hifi.

Audacity and other programs do have noise-removal facilities, but they always leave the sound messed up. The only solution is a clean recording, not post-processing.

Graham.