Kens refers to two problems he has. A slanted horizontal easiest fixed by taking the photo straight in the first place. Otherwise, Photoshop's Ruler tool will also correct it if you want to know how. Easiest is Google's free Picasa photo editing program which makes this a doddle.
The second issue as Dave has pointed out, is converging verticals or Keystoning as Americans call it. Problem caused by the camera isn't parallel to the building and likely looking upwards. The more you tilt upwards, the worse it gets. Two ways in Photoshop to correct it so the building looks straight and not falling over. Duplicate the image ( Ctrl +J). then Edit> Transform> Perspective.Drag the top corner boxes outwards and the building will start to straighten. Double click the image when done, then File> Save as.....
The Crop tool will also correct this----crop over the whole image, tick the box Perspective on the top edge bar. Go to the top corners and drag the crop so its parrallel to the tilted building edge. Go to the other top corner and do the same ( this will be dragging the box inwards to make it parallel. Double click the image and the sides should now become straight.
How much of St Pauls should be straight? I would have said all of it.
The expensive option---buy a perspective control lens. Alternatively, keep the camera back as parallel to the building as you can. Edited by: Paul Hilton at:20th August 2012 21:22