Reason #1 (95%+ of the cases) : Check the windows taskbar to make sure Investor/RT is not already running. Investor/RT can not run duplicate instances off the same database at the same time. Assuming this is not the issue, use the following solution based on your operating system.
If this is not the case, Open Windows explorer and navigate to the folder C:/Users/YourUserName/InvestorRT/admin. Within the admin folder, locate a file called "datapath.txt" and delete it. Now startup Investor/RT again. The program should successfully startup.