Why does Word 2003 automatically apply an unknown style

When I upgraded to XP and Word 2003, I immediately encountered an annoyance that was not present in Word 1997, which I had used for years.

When I type in a Word doc, I use a style I created called Letter. Any time that I have the cursor at the left margin and tab, Word overwrites my style and creates one called Letter + First Line: 3 pi. that causes me to use several keystrokes to overcome the annoyances. I certainly never created that style myself. Has anyone else encountered this and is there a way to keep Word from changing the style?
Thank you, Noel
skagitriverjournal@gmail.com

P.S. Even if I delete the bastard style, the next time that I hit the tab button, it comes back.