What version of Acrobat Reader are you running? If it's one of the later
ones (V7?) it's a known bug with interaction with Office.
The only solution I know of is to downgrade to a previous version. You're
probably not using all of the bells and whistles in the latest version
anyway...
Actually, it is a bug with 7.0 - so if you upgrade to 7.1 it should work
again. (From experience: NEVER get an Adobe product whose version number
ends in .0 )