There is a huge amount of free or very cheap audio software for Windows if you don't want to use cracks.
Do you want to record stuff you're not doing on the computer? (guitar etc.)
I second Audacity. Free (or is it GPL? available at no cost, anyway) and does the job.
Do you want to write out melodies and such and have them played back on instruments and drum machines?
You could try Buzz. If you want a more traditional looking thing there are lots of MIDI sequencers, Scytale suggested one, a cheap (not free) one that I know a lot of people enjoy is Energy XT.
Do you want to manipulate sounds, either live or recorded earlier, and maybe do some crazy improvised shit?
You could try Audiomulch or Plogue Bidule. Neither are free, but they're both cheap and phenomenally powerful and can do all kinds of crazy shit. My friend and I did a whole CD using only Audiomulch and it was awesome fun. Trent Reznor bought a PC solely so he could use it. Whatever you think of NIN (I think they're quite poo) the guy does love his production textures.
Do you find maths and/or coding (HTML or whatever) easy and want to assemble your own world of destruction from the ground up that can do anything you fucking want??!
PD (Pure Data). It would not be good if you want quick results and "musicianly" stuff. But if you want to set up a whole environment in which to either compose or perform, then this is a ... well, completely insane option.
Do you want to find your own way?
Go to
KVR Audio and use their ridiculously comprehensive search engine to find something that suits your needs.
If you want to explain more about what you want to do or how you imagine making stuff, I'd be happy to make further recommendations.