I've always loved to use Cool Edit Pro, but it is kinda expensive. Sound Forge is also good, I believe they have a scaled down version which is more affordable but enough for what you want to use it for.
Also there are a few shareware programs around, I think some people will suggest Gold Wave, but I personally haven't used it at all. If you have a Creative sound card, I do believe it comes with Wave Studio which you can also use in the meantime until you get something better.
🙂atwl