In Outlook 2003, clicking on a word while holding the Alt key brings
up a sidebar called "Research" which looks up the word in a variety of
reference sources.

Since I frequently use Alt+Tab to switch between windows, I keep
accidentally triggering this functionality. It's happened several
times this morning alone. Although I now know several things I
wouldn't otherwise (eg. "pomocnik" is Croatian for "asistent" [sic]),
it gets old very quickly.

I know how to disable shortcuts by adding their VKEY codes to registry
key HKCU\Software\Policies\Microsoft\Office\11.0\Outlook
\DisabledShortcutKeysCheckBoxes, but I don't think this helps here
because there is no VKEY code for "left mouse button".

Does anybody have any idea how to disable this feature?

Many thanks,
Tom