This is a known issue with Microsoft updates. Sometimes an update will corrupt the user.config file in PDD. If you have successfully installed and used PDD and then one day nothing happens when you try to start PDD, follow the instructions below to delete the corrupt user.config file. Once the user.config file is deleted, PDD will restart and create a new user.config file automatically.
In Windows Explorer, go to C:\Users\\AppData\Local\Pet_Diet_Designer,_Inc\PDD03.Win.exe_Url_xvyljk4223l4nzvz0bulz20uoiiqnims\0.88.3.0
Note: you will probably have a different string of characters than "xvyljk4223l4nzvz0bulz20uoiiqnims"
You should see a file called user.config
Delete this file "user.config"