Mail Format with a mind of its own..!!

I have just rolled out a corporate e-mail signature. Within that included a JPEG picture of our company logo. Everybody seemed to be fine, some had to change the mail format to HTML but there are 2 people who are having the same problem in that the logo keeps disappearing! Within the Mail Format tab in e-mail options the check box where it says ‘Use MS Word to edit e-mail messages’ keeps unchecking itself. When they go back in and put a check in the box the logo comes back, but from time to time it unchecks on its own accord.
I have tried everything, from uninstalling, repairing and getting all the latest SPacks but NOTHING works and its slowly becoming the bain of my life!!!