First, do the obvious thing and check the task bar 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.
Windows 7/8 or Windows Vista Users
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.
Windows XP Users
Open Windows explorer and navigate to the folder C:/Program Files/investrt/admin. Within the admin folder, locate a file called "datapath.txt" and delete it. Now startup Investor/RT again. The program should successfully startup.