Sophos SafeGuard Disk Encryption (SDE): cannot remove SDE after emergency decryption and uninstall

Issue
After using the'Uninstall' option to decrypt your computer's hard drive and uninstall SDE, the program still appears in Add/Remove Programs in the Windows control panel. When you attempt to remove SDE, you get an error message saying that you can only remove a program if it is installed.

Sophos product and version
Sophos SafeGuard Disk Encryption

What to do

Occasionally, an emergency decryption and uninstallation of SDE fails to remove all components of the program.

Reinstall Windows to ensure that the hard disk can be successfully re-encrypted.