This applies to XP computers only, and if you use the messenger service chances are that you have gotten confused about the difference between the two different Messenger names.
Windows Messenger (WM) is part of the XP operating system while MSN Messenger (MSN) is another Messenger with more bells and whistles than WM.  If you are happy with WM you won't have a problem but if you prefer the MSN one then you have to get the WM to stop loading when you start your computer.  While they both sign into the same service some folk prefer the extra features of MSN.
Sometimes WM is a bit stubborn and loads even after you un-tick everything.  Try this anyway.  Open the main window of Windows Messenger and go to Tool/Options and remove all the ticks to do with WM loading when you start windows as well as logging on to the internet.  About 4 ticks in all.  Click Apply then Ok. 
Open Outlook Express and go to Tools/Options and under the General tab and remove the fourth tick by, Automatically log on to Windows Messenger.  Click Apply and Ok.

Another cause for Spell Checker not to work in Outlook Express is if you have a different Language selected in Word to what is selected in Control Panel / Regional and Language Options.  These must match for the spell checker to work in Outlook Express.  

Option Two
 

Warning:
Before changing anything in the registry back up the registry.
After opening regedit, click File and export and give the file a title like the date and save the file where you know where to find it.
Tip
To save having to try and find the big long string, Copy the string between the { } then in regedit, click Edit and find.  Paste the string into the window and click Find Next.

Sometimes removing or disabling Windows (or MSN) Messenger causes OE to open slowly. This will be shown by an entry in the system error log:

The server {FB7199AB-79BF-11D2-8D94-0000F875C541} did not register with DCOM within the required timeout.

To fix this, open Regedit and navigate to this key:

HKEY_CLASSES_ROOT
\CLSID
\{FB7199AB-79BF-11d2-8D94-0000F875C541}
\InProcServer32

In the right-hand pane, double-click on the (Default) value and delete whatever is entered there, leaving it blank. Click OK. Now perform exactly the same step with the key directly underneath, namely

HKEY_CLASSES_ROOT
\CLSID
\{FB7199AB-79BF-11d2-8D94-0000F875C541}
\LocalServer32

Once both (Default) values are empty, Outlook Express will open normally.

 It worked a charm for me.

Spell checking does not work and cannot be enabled in Outlook Express 6.  When you try to spellcheck a message, you may receive the message "An error occurred while the spelling was being checked."

Outlook Express does not have a spell checker. It can make use of the spell checker in Microsoft Office 95 or later, or Microsoft Works (later versions, which include Microsoft Word) if present. However, a system file that is required in order to make use of the spell checker is not installed by Windows XP.

Download a copy of the file by clicking on this link. Just run the downloaded file to install it into its usual location. Be sure to save the download to a place that you can find it.

Also check:
Open Control Panel, Regional Settings. Check under Regional Options that the language selected is the same as that used by Microsoft Office, i.e. English (United States) or English (United Kingdom).

Open Outlook Express, Tools, Options, Spelling tab and check that the language selected there is the same as selected above. (One reader from an English-speaking region outside the US found selecting English (United States) solved the problem.)
 

If spell checking still doesn't work for Outlook Express after you've done all this, then unfortunately we are out of ideas. If you should discover any other reasons for this problem, please let us know so that we can add the information to this page.