Office 2007 - mail merge errors with DDE connection

I am using Word to generate email messages to distribute through mail merge
and the data sources are Excel files. I am having problems when I try to
bring in the Excel file - message comes back saying "Word could not
re-establish a DDE connection." However, I had just completed on mail merge
and successfully sent the emails but when I tried to bring in a new Excel
file, the error message appears.

All the Word documents and Excel files are on my single laptop - no network
or server involved. The Excel files have not been opened "Exclusively" and I
have tried restarting my computer to see if they work but still get the same
error message.

I am using Vista Professional with Service Pack 3 and all the latest Windows
/ Office updates have been installed.

I need to use this email (mail merge) program quite frequently and it is
becoming a very frustrating experience when it does not work, particularly
when I just sent one batch of emails and then the next one will not work.

Any suggetions are welcome.