First thoughts are Paint.Net and The GIMP. I have yet to find anything "simple" as most are generic paint programs.
It sounds to me you are really looking for FORM FILLER apps.
I have Windows XP sp3 and a very very old version of Snag-It to take screen prints and make jpg files. I want to add arrows to point out items on it. I'm using Paint but it doesn't have an arrow so I'm trying to make it myself. That's all I need - something simple just to add lines and arrows. Any recommendations?