If you go to (for Office XP = 10.0, Office 2000 = 9.00 i think)

HKEY_CURRENT_USER\Software\Microsoft\Office\10.0\Outlook\Options\Mail

the key "EditorPreference" I believe is for each editor. As far as I know the values are as follows (in Hex).

10001 = Plain Text
30001 = Rich Text
20001 = HTML

Therefore just change this key from any 20001's to 10001. This won't require local admin rights.

Oh, and please test on yours, worked ok on my quick test.
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---------------------------- Diane Confucious say: Woman who keep washing liquid on top shelf, jump for joy