I guess the question is why is Notepad necessary?
Generally speaking, the NotePad main edit window has a property available in TestComplete. You can obtain that value, parse througn it using aqString or other similar methods to find what you're looking for, grab that text, and then set the property in another Notepad to contain that value.
But as Marsha_R, using Notepad only makes sense if your application under test requires it. Is the file you're opening in Notepad containing data that is used in the test? If so, you don't really need to open the notepad window at all... you can use aqFile.OpenTextFile method to open the text file and operate directly against it's contents without having to involve the notepad application at all.
so... if you can tell us a bit more (perhaps with samples of the notepad files you're wanting to manipulate) of what you are hoping to achieve, we can give better help for you.