I've been working on a small example that should work for you. Microsoft has developed SDK's for this purpose.
This VI will speak the string that the user inputs. The user needs to have Microsoft Speech SDK's and the Microsoft Speech Engine installed. For more information, go to
http://www.microsoft.com/speech/
or check out
http://msdn.microsoft.com/downloads/default.asp?url=/downloads/sample.asp?url=/msdn-files/027/000/781/msdncompositedoc.xml
Before running this LV code, make sure you visit the speech site and download the newest Speech SDK's. I think its 5.1. I've tested this on a Win2k machine.
Best of luck
Bilal Durrani
NI