When we installed and tested Flexi Pro v8.1 on Windows 7 64-bit RC we did not have any problems with the installation or running the application at all. Check if the user account you are using to install the program has administrative privileges or if it is a limited user account, as yes you will need an administrative account. The user account we set up the installation with was the only user on the computer and thus an administrative account. After the application was installed we did not have to start the program using the "Run As Administrator" option, it just ran fine as normal.
It's hard sometimes in situations like this to figure out what errors you are running into. Is it a driver issue with the Sentinel key driver? Is it a firewall or security program issue? A better description of exactly what you are doing and what the installation is doing and not doing would help try to solve the problem.