When formatting a list with bullets, my text changes to weird symbols. In the last document, all of the text in bulletted lists suddenly changed to symbols. What can I do?

Check the style definition for the paragraph style you have applied with the bullets. What font is set for that style?

I think you figured it out! The font automatically changes from Times New Roman to Symbol, which I have never heard of...
Do you know what causes this to change on its own, and how I can prevent it?
Thank you!

There is a style optional setting called "Automatically Update" which can lead to this behaviour. Have a look at the bottom of the Modify Styles dialog for a check box with this label.

Note that you won't see the setting on the Normal style in Word 2007.

I was finally able to fix this formatting with your suggestion. Thank you so much!