Simultaneous use of MS Word and Excel 2007

I work for a certain non-profit organization and I have been given the task of writing a letter using MS Word 2007. This is not an ordinary letter as it will be configured to bear the names and addresses of around a thousand donors to our organization.
Since I have a spreadsheet containing all these details in Excel 2007, I am wondering if there is some way to print out a letter containing this info in Word at the same time avoiding copy pasting the details on the spreadsheet for each letter as this would take an eternity.
I am counting on your help.
