The Sophos installation error may result due to the leftovers of the previously installed programs or corrupted files. Usually, when you use a third-party uninstaller, it may not be able to delete all the remnants to uninstall Sophos. Also, the problem may arise when another security software is installed, or your system files are infected.
Check your system to see if the third-party security software is listed in Programs and Features. If yes, the problem may be tamper protection enabled in the software or the corrupted software. In order to get rid of installation issues, you will need to remove/uninstall the software and enter the required details if prompted. Complete the uninstallation process. Once the uninstallation completes, remove the tamper protection on the third-party software. For this, you have to contact the vendor. On the other hand, if the uninstallation fails, it shows that your installation is corrupt.
If third-party software is not listed in Programs and Features, you need to detect the remnants of the previous software causing the issue. Find what's causing the detection via AVRemove.log. Launch the log with the text editor and find the log from the bottom upwards. Remove the detected products and check the items mentioned below the text.
The registry keys beneath the text are the leftovers of a previously uninstalled third-party software product. Those keys need to be removed manually from the computer's registry.