My computer crashed in the middle of a defragmentation run, corrupting just about everthing that was running at the time (so, don't have anything else running when you do a defrag!).
After recovering most of the corrupted stuff, I was left with a problem with Outlook Express, in that it showed an address book error, refused to send emails, etc.
Eventually, I worked out that it was not Outlook Express itself that was the problem, but that the Windows Address Book was corrupted. Re-installing Outlook Express does not help (it does not re-install the address book - if it did, you would lose all your addresses!).
The address book has an extension .wab (in my case on XP it was in c:\Documents and Settings\User\Application Data\Microsoft\Address Book, as User.wab).
To rectify the problem, I had to delete the corrupted version and replace it with the one from my older computer, then re-boot.
My suggestion is that as a safety measure, you should take a copy of the .wab file, and store it elsewhere on your computer or on a disk. Note that this is NOT the same as saving your address book contents as a .csv file from Outlook Express (although you should periodically do this too, to keep up to date) - if the .wab file becomes corrupted you will not be able to import the saved addresses back in!
Hope this helps some of you - it only takes a couple of minutes to copy the .wab file, and could save you a lot of grief!