Fradu, this sounds like you have some file corruption within Outlook Express. I'd suggest un-installing it, then putting it back again. Please remember to backup your data files, usually found in the C
WINDOWS\Application Data\Microsoft\Out...\Mail folder, with the extensions .mbx and .idx
Once re-installed, you can copy back your data and the problem should go away. The only other possibility that comes to mind here, is that you may have been clobbered by a virus.
Hope this helps