Thanks for the suggestions
I should make clear that both my operating systems (C, the one I'm using and, D, the one I want to remove) are in separate partitions on the same disk.
I have tried to delete D, but again, I am told I am not allowed to delete system files or am just unable to do so. Also since trying that, I am getting boot errors ("bootmgr is missing"). Consequently I have been having to use the installation disk to repair and boot up. There was no poblem with it until I started trying to remove the unwanted operating system. The system is now acting like it is on its way to crashing. I do have backups fortunately.
Any other ideas would be appreciated.