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!
Joe

Corrupted .wab file:
p.s. - If your .wab file is located elsewhere, you can easily find it just by searching for .wab. It is probably the only one on your computer. If not, why not also make copies of the few other ones too!
Joe

my best advice, use mozilla thunderbird, it's faster, and has a smoother looking and user friendly interface.

Go Mozilla!