Windows Installer always says "Another program is being installed"

Solution 1:

Open regedit and see if this key exists:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Installer\InProgress

If so, remove it. Restart the Windows Installer service after removing the key.

Solution 2:

Fire up Process Explorer and hover your mouse over each instance of msiexec.exe. With a bit of luck you'll get the command line of what's installing which should give you some clues as to what it is.

I get this from time to time and it's my antivirus installing something which I can only guess is new definitions, because it happens frequently enough for me to notice.