Message "Another installation is already in progress, you must complete the installation first" prevents installation. Rebooting does not help.
There have been cases with STAAD & RAM Products, when user is trying to install the product, he/she gets this message. This occurs when the first time user tries to install the product, it fails or remains incomplete.
To resolve it in a Windows 7 machine, go to Start, Right-click on the Computer icon, and select Manage. This opens the Computer Management utility. Select Services, then right click on Windows Installer in the list and select Properties. If you’re having the problem, the Windows Installer service is started, and the Stop button is disabled.
In the Properties dialog, change Startup type to “Disabled”, click OK, now reboot your computer. This stops the service. After your computer reboots, again open Computer Management, open Windows Installer Properties, and change the Startup type back to Manual. Now you will be able to start a new installation.
For other operating systems, look for the services and follow the steps as mentioned above.