I have now got my APBackup working again, by doing the following: -
I changed my D:/Programs/APBackup folder to APBackupOld, and then uninstalled APBackup version 3.8.5983, listed in Windows Vista/Control Panel/Programs and Features/Uninstall or change a program.
I deleted the C:/ProgramData/APBackup folder manually, because it was still there.
I then re-installed APBackup into D:/Programs from my original download. I had been reluctant to do this, because I thought all the Tasks I have set up would be lost. However, on opening the new APBackup. all the Tasks are still there, so I can now carry on as before.
Can you please tell me where the data for the Tasks is stored. I thought it was stored in the ArhTasks.bpd folder, but clearly it is not, because I had deleted that folder when I deleted C:/Programdata/APBackup.