How do I hook into other programs in Windows?

Question

Can anyone explain how does one program hook into and modify behavior of other programs in Windows?

How is it even possible? Don\'t windows programs protect themselves

Answer 1

Imagine an application that saves data to file X.txt you can grab the x.txt contents, and attempt to find a difference in the saved x.txt against the current x.txt, once it changes you can have an event fire knowing that program X modified its x.txt file.

You can do this on a lower level but the concept remains the same, (monitor something for change).