Install MorphVOX. It allows you to change and morph your voice on-line.
It doesn't matter the program you are using, it supports IM programs (Google talk, MSN, Skype,...) and games.
You'll be surprised by the effects that MorphVOX can produce.
Now you can change your voice automatically. It includes, for example, a girl,a boy, a nymph, a man, a dog or the devil.
It also includes several settings to modify more sound features.