Mmmmmmmmmmmm.... Tricky!
I would stop
mooning if I were you.
Sorry, not helpful, but I couldn't resist.
Apart from uninstalling / re-installing Office / Word (or doing a repair), ensure that the keyboard and locale are correctly set.
Check if there are any macros with "M" created as the keyboard shortcut. Ensure that auto correct / auto type is off, and Smart Tags are off.
Is it "m" or "M" -or both - that causes the problem? Has it always been this way, or only since a recent event? Can you trace the "event", if so?
Finally, ensure that all the service packs / updates for Office 2K are installed