any freeware that enables.. Options

[clarity]

Well okay so me and my friend are making a short type skit/movie and she's suppost to be a boy.
But she's a girl, so of course she doesn't sound like a boy. Anyways in the skit/movie we want her to sound like a boy. I think we'll probably do voice overs and sync it with the lip movement. But any who is there a program out there that enables you to change your voice when you input it in the computer or something?

and preferably a program thats free.

thank you in advance.