You mean open the letter in Word from Access? First, check that the path ends in .doc as expected. Then simply run the file using the VB shell command. Windows will look up the application based on the file type. That's the dead simple way of doing it.
Alternatively, you could do some COM scripting, which is a little more involved.
Set objWord = CreateObject("Word.Application")
Set objDoc = objWord.Documents.Open(path)
rem do stuff with objDoc
rem when done:
You could leave it at simply opening the document.