"computer voice" software?

I need a highish quality program (read: it sounds good) that can generate a computerized voice.  It can be text-to-speech (I type, it talks) or it could even be something that takes my own voice and modifies it (a-la GLaDOS in Portal).  Anything that will mask my voice and turn it into something electronic.  I already tried one (it was just called "Text2Speech") and it kind of sucked.  I'd like something that actually sounds good.

And no, I'm not plotting anything illegal.  I'm making a video for school and I want the effect.

CoolEdit used to have a feature that modulated/masked a track with another track, for example a voice track with a music track, when it was still called CoolEdit. Nowadays the software is called Audition (Adobe).

This flash animation uses this feature:
http://dagobah.biz/flash/Cyborg_Jesus.swf

The guy recorded his voice and a guitar track and used the latter to modulate his voice.