you can hook up a midi keybord, or a keyboard with usb or midi out, to the free audacity, then you can download some free dx or vst plugins etc and install it into audacity (or another host), to give it all kind of sounds.
I have an Edirol midi keyboard connected to my pc through USB, and I also have a Roland G-800 arranger workstation hook'd up through its midi out port. I am using a M-audio Delta 1010 soundcard to hook it up. Host application is cubase, but audacity has support for plugins as well.
Using a midikeyboard and a softsynth, you can play all sort of instruments like, drums, cello, guitar, piano etc... if you play piano or know music notes and theory, you can program many of these synths without a midi keyboard
There are a lot of free plugins to choose from, just google around a bit.
You can change your voice in a similar way, just dl a free EQ, delay chorus compressor, pitch shifter plugin and start to rock, or get ohmboys and start tweaking
There are a lot of free software for all this, so there is really no need to spend a truckload of cash when you are first starting out