We recently got this case into Product Support....
"Basically an update for Sophos Antivirus can cause an instance of the Windows installer to run in the background. Since Windows will not allow two installers to run at the same time, anything that uses an MSI will result as an error that usually says something to the effect of “there is an installation running, please wait until the install is complete”.
This occurs in both cases of trying to uninstall and reinstall. When trying to install from the Design Suite (original media) everything will look as if it is going to work out fine, you can put in your serial number and set up your licensing etc. but at the end splash screen just shows a FAILURE with a big red X in the checkbox. When you check the log file in the temp folder, the error kicked back is a 1618.
The fix is pretty painless once you figure out the problem [I can’t take credit for the solution]. Basically you download the script from Sophos, on the page referenced below, manually kill the Windows installer in task manager, and run the fix….and viola!"
Thanks to CJ Sondgerath for sharing this tip with the community.