Can anyone recommend a software to me that can be used to produce a computer voice narrator for a short promo video that doesn't sound totally embarrassing?
You can check out Cepstral. They have desktop software, and online services for Text To Speech. They are about as good as you're going to get, there are lots of different voices available, means to alter articulation, etc. And you can "demo" the quality of their technology online (you enter the text, select the speaker, and it will generate the sounds for you to listen).
However, all text-to-speech technology today sounds rather robotic. If English is not your native language, and you have a strong accent, then maybe it's better than you talking yourself, otherwise your own voice will probably sound better.
As an alternative, there are rather inexpensive "actors" that will do a voiceover for you. I personally ordered some clips from Cheap Radio Spots in the past and was very satisfied with the quality.