Hi,

This usually has one of 3 causes:
1 - Another MSI installer is currently running. You've already restarted Windows which should rule that out, but you would need to be absolutely certain. e.g. is Windows update installing updates in the background.
2 - A registry entry has been left in place by another MSI install (usually when an install is killed by the user in task manager)
3 - The MSI installer service isn't working correctly.

The only fixes I'm aware of:
For 1 - Check for any running install processes - msiexec.exe in the task manager.
For 2 - look for and remove an InProgress key under HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Installer if there are no msiexec.exe or install processes.
Usual warning applies here to be very careful when making any registry changes and make a backup.
For 3 - Check the status of the "Windows Installer" service. Bring up the services panel - services.msc from the run dialog. The service should be set to manual and after a reboot probably won't be running. Make sure that you can start/stop this without it reporting errors.