First, check that Internet Explorer|Tools|Internet Options|Programs (or the
equivalent in older versions of IE) has Outlook Express set up to be your
default e-mail client. It almost certainly is.

Assuming that isn't enough, you may be able to fix this by adding some lines
to the WIN.INI file, which should be in your Windows directory (typically
C:\Windows)

To try this, proceed with caution! Close all your programs (Word, Outlook
Express at least), and open the WIN.INI file with Notepad and ensure the
following lines are in there, or add them if they are not:

[Mail]
MAPI=1
CMC=1
CMCDLLNAME=mapi.dll
CMCDLLNAME32=mapi32.dll
MAPIX=1
MAPIXVER=1.0.0.1
OLEMessaging=1

(IN fact

[Mail}
MAPIX=1

should be enough, but the above lines are the ones you will typically find)

Please note that even if this procedure works, it does /not/ fix the other
problem where the Outlook Express address book cannot be used as the address
source for the merge or in the Envelopes and Labels dialog box.

--
Peter Jamieson - Word MVP