I tried it to simulate your problem. I create new project from folder with some text files. When i doubleclick on TXT file, file opens in PSPad. No Notepad, no other instance.
Go to PSPad Program settings / System and check if TXT files are set for opening in PSPad. -- <http://forum.pspad.com/read.php?2,51011,51031> PSPad freeware editor http://www.pspad.com
