See http://support.microsoft.com/kb/198299 "You receive a 'Word
could not re-establish a DDE connection to Microsoft Excel' error
message when you attach a Microsoft Excel worksheet as a
mail-merge data source in Word 2000".

Eric_G wrote:

I am attempting a WORD merge using an EXCEL file as the data file; the data
file is quite large -- over 7500 rows / records. Each time I try to specify
the EXCEL file as the data file, I get the above-noted error message. Is
there any way to overcome this problem. I'm only assuming it has to do with
the size of the EXCEL file but am not sure.