I had same problem upgrading from 14.2 to 14.3. The install failed installing SMWebSvr. I am also using IIS, so just like Antony, I don't need the SMWebSvr to be running. The problem seems to be that when it tries to install the files for the service it fails because the service was not removed. Sadly the rollback also removes the previous 14.2 files thus leaving the Smarter Mail completely dead. More sadly is that it removes the files for the service but leaves the service configured. So the SMWebSvr service shows up in the services list but you cant do anything with it there because the executable files are now gone.
The solution is to manually remove the SMWebSVR service before the install (on my system a reboot was not required ... Windows server 2012). The service can be removed by using: "SC delete SMWebSvr" at a command prompt. After this is done the 14.3 installer works fine.
This is clearly a bug in the installer that has existed for some time.
And the rollback is way too aggressive. It removes files that it overwrote during the install thus removing the files that you needed to run the previous version. After rollback your system is dead anyway so why bother with rollback.