This gets a little complicated.

Until Office 2003, the spell-checking for Outlook Express was provided
by Office. That's no longer true in Office 2007. That means you'll
have to reinstall the Office XP spell-checker on your Vista box to do
the job for OE (see
http://www.outlook-tips.net/archives/2006/20061228.htm). That version
can continue to use your current custom dictionary.

For Office 2007, you need to put a copy of the custom dictionary in a
new place. The file name is still Custom.dic and the format is the
same, but it's a different folder.

First find the existing Custom.dic: In Windows Explorer on the old PC,
enter the address

  %appdata%\Microsoft\Proof

(The "environment variable" %appdata% resolves to the correct
Application Data folder on most versions of Windows, including XP and
Vista.) Copy the Custom.dic file to a USB drive, floppy, etc.

Now go to the same folder on the new PC and paste the file for use by
the Office XP spell-checker. Then go to the folder

  %appdata%\Microsoft\UProof

and paste a second copy there for use with Office 2007.

Unfortunately, these two copies aren't connected in any way. You'll
have to manually add words to both dictionaries, or copy the file from
one place to the other when one changes. A tool that can help with
this is SyncToy
(http://www.microsoft.com/downloads/details.aspx?FamilyId=E0FC1154-C975-4814-9649
-CCE41AF06EB7).

--
Regards,
Jay Freedman
Microsoft Word MVP        FAQ: http://word.mvps.org
Email cannot be acknowledged; please post all follow-ups to the
newsgroup so all may benefit.