I use Photoshop, but hopefully PSP can do what is required. In addition to the 'sloping horizon' The image has pronounced barrel distortion and also 'keystoning' (the latter due to camera beng tilted up).

I corrected the slope (you can use 'rotate image' or do it in the lens distortion filter), then corrected the barrel distortion (lens distortion filter) then adjusted the keystoning to get the vertical alignment correct(edit, transform, perspective). finally cropped image as all that transformation resulted in a less than rectangular image area.

I did this quickly, obviously with a small file (still not sure the verticals are!), but here goes...