Try getting Adobe Acrobat to re-register itself with windows. If you go to Start > Run and input:
Acrobat /regserver
that should do it, though you may need to insert the full path name as well. For example (for Acrobat 8.0):
C:\Program Files\Adobe\Acrobat 8.0\Acrobat\
Acrobat /regserver
and, if you've only got the reader installed, I think the executable's name is 'AcroRd32', instead of 'Acrobat', so you'll need to use that instead.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]