Well, actually it is easiest if you get them straight in camera. Those who use PhotoPaint and have seen PSP or LightRoom in action can develop Horizon Correction envy. It is actually quite easy to correct horizons in PhotoPaint without scripts or complex calculations.
I have put a tutorial (1.4 MB Video) HERE
I hope it is useful.
Dang, Nab It! Now I will have to go through my hurried snapped photos and straighten them out, using your method.
(Ratzel-fraz-rata-ratzel!)
Sometimes you end up with a badly tilted photograph for which you have not enough margin to be able to rotate the full amount. Don't get frustrated, just go to page size and expand it a half inch or so both in height and width. Now you have enough real estate to rotate your crop to get it done all in one click.
Best advice in all situations, is that frustration adds nothing to the job except extra time. Job one is to stay happy and composed.
I think everyone thinks clearer that way.
But there are times when leaving PhotoPaint out of the loop is what I want to do. In such case: crooked business card scan, the three-point rectangle tool can be made to work like the PaintShopPro horizon straightening too. You pick out the long side of the business card, draw your first two points, remembering to drag to the second point. then to the height of the card. PowerClip the bitmap into the newly formed container. Double click the PowerClip to get rotation handles, move your rotation axis to the corner you want to use as zero degrees, then zoom in, make sure snaps are on to high, make a guideline common to the zero degree corner and rotate to the guideline. With a little practice, this works fairly fast.