Exploring XP 64 bit I find you have 2 separate Program Files folders:
C:\Program Files - for 64 bit apps. & C:\ Program Files (x86) - for 32 bit
apps.
Both have an Outlook Express folder and if either copy of OE is run any
attempt to spell check gives the error.
It seems both the copies of Outlook Express included in the standard install
are 64 bit and therefore don't permit a spell check using standard 32bit
spell check software.
64 bit spell check software doesn't seem to be available yet.
Anyway thinking it through I tried the following:
Using safe mode from startup both existing OE folders were renamed to BAK
OE.
The OE folder and content from a normal XP 32 bit install (different PC) was
put in place of the 2 renamed folders.
All now working ok - A Spell check at last.
Why Microsoft couldn't solve this somewhat mystifies me....
They are certainly aware of the problem as it's documented in there release
notes for XP 64bit.
Anyway hope this helps someone else.