Yes, it is possible to remove an outdated operating system installation from a partition.
Yes, it is possible to remove an outdated operating system installation from a partition.
The reason you can't finish this process is because the boot files are stored in that specific partition. The simplest solution would be to use your Win8 disc and remove all partitions. Why do you need to install both Windows 7 and Windows 8? Are you upgrading to a newer edition? Another possibility is connecting it to another device and wiping the partitions there.
I’d go with the first choice—save any files in the Windows installer, wipe all partitions, rebuild them as desired, and then install Windows. Avoid connecting it to another device and removing partitions from there, since that could erase boot files and ruin the setup. You’ll lose everything if you do that.