Perhaps the installer is looking at a registry key.
In regedit.exe, navigate to
There should be a key with the name,
InProgress. Delete it.
(As far as I know, deleting this registry value is quite safe. But be careful nonetheless!)
Here's how to open the registry editor in Microsoft Windows.
In the Start menu, click on "Run". Type "regedit" into the one-line box and click "Open".
You can access the registry editor, regedit.exe, by clicking on the Start button and searching "regedit". The first result should be a blue cube-looking thing. Click it.
In the Start screen, search for "regedit". The first result under "Apps" should be the registry editor. Click it.