outlook 2003 suddenly converting files to winmail.dat

i have been all over these discussion groups and have not found an answer
that solves my problem. i have been using outlook 2003/vista for my email for
5 months now with relatively few issues. on friday, i was sending emails
with .doc attachments to 2 people (editing & proofing via internet). one was
on a corporate email account in s. korea and the other using aol. we were
commenting on said files via live messenger. i resent these edited files an
hour later and both recipients could not read them - winmail.dat. I have
checked settings: outgoing email in html (no change), each recipient -
outlook to choose best way to send (no change). i've switched to only plain
text with the same results. i can send to some people without this happening
(who do not use outlook to receive), but overall this is now a standard
problem. what has me confused is why it would start up suddenly and why
nothing i do to corrects the problem. could it be something i clicked on
accidentally that night? could it have to do with an increasing volume of
saved email in outlook? anyone?
--
gnp