Try to view your images on 100% zoom level.
100% is often to large for a working file although useful for selective editing of specific parts of a photo. Try viewing at 25% , 50% or 75%.
You can do this quickly by holding down the Ctrl key and pressing the plus or minus keys on the numeric keypad.
Thank you for your information about my problem with PSE 11. I have since tried the programme on 2 different computers and it works fine. Seems like the problem is with my computer although my old PSE 10 works okay. I'd appreciate any ideas about what could be wrong with my computer to cause my photos to break up into various segments which don't join together particularly when I move them into 'before and after horizontal' or '......vertical' and even when zooming them. ( HP Pavilion Elite) Vista Ultimate.
Try resetting the prefs. From the Editor menu click:
Edit >> Preferences >> General
Then click the button “Reset Preferences on next launch”
Close and re-start.
Thanks for the information. I tried resetting my preferences but no luck. I even updated all my drivers which resulted in more problems with my computer and didn't solve my PSE problems. I can't think of anything else and so will have to use the programme on my laptop, which is also Vista by the way, but not Ultimate.
Thanks again for your suggestions.