Highlight the file and then hold down shift and right click it. You should have the option showing "Open with" You can also go into PSP, File, Preferences, File Format Preferences and unassociate the file extension in there.

"It is good to see ourselves as others see us. Try as we may, we are never
able to know ourselves fully as we are, especially the evil side of us.
This we can do only if we are not angry with our critics but will take in good
heart whatever they might have to say." - Ghandi