Under Tools in Explorer go to Folder Options. Go to File Types, Scroll down to the entry for TXT and select it, then hit advanced. Select Open (or whatever action is in bold) and click the Edit button, then paste this in under "Application used to perform action":
C:\WINNT\system32\NOTEPAD.EXE %1
Make sure Notepad.exe is actually in C:\WINNT\System32. If it isn't just use the path where it is.
Oh yes, if none of that works go the TXT entry in Folder Options again, go to Advanced, select Edit in the list and hit the "Set as Default" button.