I believe you are looking for a text-to-speech synthesizer.
Windows 2000 and Windows XP come installed with a text-to-speech synthesizer.
Now, making your computer talk? Well, that is a different story all together, but i think that you will find that the built in synthesizer will work well for word documents or chat rooms.
Here is a link to a site that is a review of several different types of text-to-speech synthesizers. You will find that if you search for them by name on the internet you will find ways to download and use them.
http://www.student.oulu.fi/~vtatila/reviews_of_speech_synths.html#speechreviews
Here is a link to download a few of the synths, their voices and some different languages. Im sure there are more comprehensive lists of these somewhere but i think you can get the idea from here.
http://www.bytecool.com/voices.htm
Hope this helps!