I've seen reports of mails disappearing (which is nearly the same thing as
you describe) but I've never seen a surefire explanation. Some things you
can look at/try
a. can you still use the appropriate Word File|Send To option to send a
single non-mailmerge word file?
b. are you mailmerging using HTML format? If so, can you try a small
experiment using merge as plain text or with an attachment? If you see the
security dialogs that Outlook pops up, that at least tells us that Word is
trying to do something and Outlook is trying to respond.
c. does your copy of Outlook have any COM Addins installed? See Outlook
Tools|Options|Other|Advanced Options|COM Addins. If so, can you establish
whether or not it is OK to disable them temporarily, then try doing that and
see if that makes any difference?
d. It may be worth running the Outlook data file repair utility called
scanpst.exe. On my system it's in

C:\Program Files\Common Files\System\MSMAPI\1033

e. as long as you have your Office XP CDs, you can try a Check and Repair.
(Personally, I'd always try to avoid a full re-install but I suppose it may
come to that - and you might end up re-installing everything, Windows,
McAfee, the lot).

I doubt if I'll have any ideas beyond that, but if you do find the solution,
it would probably help others if you could post it back here.

Good luck!

Peter Jamieson