sinthetic voice

i was wondering if anyone knew a free program (or if i must pay a big name program that would be sold in the local softwere store) that would say what i tiyped... like i type in "hello and welcome to my computer" and it says out my speakers "hello and welcome to my computer"
NOTE: im planing to use it with another program that i got that on computer login runs another aplication... i need a program that i can personalize to all the people that use the computer