Could be the Windows Installer is not working correctly.
See " Check Windows Installer" section part way down the following thread.
Program update does not complete - eM Client
If the Windows Installer appears to be running ok, then Try downloading the Windows Troubleshooter
Fix problems that block programs from being installed or removed (microsoft.com)
Note:- Make a backup of eM Client if you can still access your current version via "Menu / Backup.