Important: You can get this message for two reasons.
Reason One:
When attempting to upgrade to Sophos Anti-Virus for Mac version 9 from version 8 you will see the error message shown below (in the version 9 installer) if you have previously removed the version 8 uninstaller.
Unlike some self-contained Mac OS X apps your cannot simply drag the 'Sophos Remove' app to the Trash to uninstall. You must either run the remove app or, if you want to upgrade straight to version 9 leave it in place so the version 9 installer can make use of it.
To get pass the error shown above you need to restore the 'Sophos Remove' app from your Trash folder and run the verson 9 installer again. If you no longer have the 'Sophos Remove' app you can download a copy from the link below.
After downloading the file you can double-click it to extract the app file. Then copy/move the app to your 'Applications' folder. You can then either uninstall version 8 by double-clicking the remove app from within the Applications folder, or simply run the version 9 installer which should now proceed pass the point of failure that occurred before.
You may want to watch/listen to the video below that talks about the process further.
Reason Two:
Hold on! There is a second reason for the error An incompatible product is installed to be shown. - you are trying to upgrade an installation that is not the Home Edition v8 to Home Edition v9. You probably were given the installer by your company or college and not from sophos.com and it wasn't the Home Edition.
If you open Console, expand /var/log, and select install.log, you may see The "opm-v8" product is already installed. Remove the existing product before installing the "he" product.
You will not be able to directly upgrade v8 to v9 if version 8 is not the Home Edition. If you originally got version 8 from your company or university then it could be the 'corporate' edition that they gave you and hence you'll experience this problem.
You'll need to first remove the current version 8 using the uninstaller. You can locate the uninstaller through Finder and browser to the /Library/Sophos Anti-Virus/ folder, then run the Remove Sophos Anti-Virus.pkg file...
Then re-run the installer for version 9 of the Home Edition. Watch the video below for more information