Never mind my previous answer. I figured it out.
1. Format—> Style (this is from the menu at the top)
2. That brings up this. Select modify after selecting the Normal theme.
3. That gives you this. Select Times New Roman as the font. Make sure you check Add to Template. Select OK.
4. Hit Apply on the style menu in the first screenshot.
I hope that works. I tested it in Word 2008 on a Mac. The next time I started Word the default was TNR.