Macs come with Garage Band. I think you download a program for PCs called audacity. It's free!
Garage Band has some EQ effects built in that can modify your voice. I bet Audacity does too...at worst go to the toy section of Target. There are all kinds of voice changers there. From Ben 10 toys to Tranformers helmets. You could just record while using one of those.
Those are very simple and inexpensive ways to get started. For some dough, Mike Fox (Fox Haunt and Skulltronix) or Rusty Knife would be glad to assist you I bet.