I'm not sure how you are writing out to the text file if it's as simple as an export, or the I/O Open statement or if you have some VBA that performs this, but ultimately if you can store the path you are writing to, then the following should work.

[VBA]
....
'Set filePath to the txt file path
Shell "notepad.exe " & filePath, vbMaximizedFocus
[/VBA]