See http://word.mvps.org/FAQs/AppErrors/ReRegisterWord.htm

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Suzanne S. Barnhill
Microsoft MVP (Word)
Words into Type
Fairhope, Alabama USA
Word MVP FAQ site: http://word.mvps.org
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"TOZ" wrote in message
news
Following a "complete install" upgrade from Office XP to Office 2003 Pro
MS Word no longer appears in the "Open With" dialog that you use to
select a program to open a file when no association exists. Even if
you browse for the winword.exe file itself it will not allow it to be
added to the list and will not open Word. You can find the winword.exe
file
and select it but doing so simply returns you to the Open With dialog box
and
nothing happens. Nothing I have tried will work. Some systems we have
migrated do not suffer from this but many others do. No idea what makes
it
happen to some and not to others. REALLY need to solve this. Thanks!