How can I add to the Microsoft text-to-speech pronounciation dictionary?

Is it possible to add pronounciation keys to a text-to-speech dictionary? The one supplied by Microsoft (XP) can already understand common abbreviations. However, I quite often use abbreviations such as dH2O for distilled water. The computer can say "distilled" and "water" but how can I ociate dH2O with "distilled water"?