Open up a folder and then go to the menu:
Tools > Folder Options > File Types
Go down to the TXT extension and then click the Advanced button for the Edit File Type dialog:

If you are missing the open action
To create a new open action, take the following steps:
- Click New. A New Action dialog will pop up
- For Action, enter
open
- For Application used to perform action, enter
"C:\WINDOWS\system32\NOTEPAD.EXE %1"
- Click OK

(Adjust the program file path as necessary if you prefer opening with another text editor.)
In the Edit File Type dialog:
- Select open under the Actions list
- Click Set Default
- Click OK
- Close out the Folder Options
This should now set back the default action of double-clicking on a .txt to open instead of printing.