Block the Screen

I have a business where I lend computers to customers for a certain amount for a specified number of hours. Mostly, customers rented my computers for gaming (Counter Strike, Quake 3....). I am maintaining 15 computers manually and I found it hard to keep track on the usage of the computers. What I want is that I'll make a program that is going to block the screen after the specified time (No. of minutes/hours) automatically, thus preventing the user to continue using the computer if his time is up. I created a program with such purpose by using the SetWindowPos API. I was successful when the user is only using windows apps. such as MS Office or the internet. But in terms of games, when it attempted to block the screen, all I get is a blinking screen and the game application prevailed. Is there any other API to use to solve this problem or is there any free program with such purpose whcih I can download?

Take a look at your other thread, it's a suggestion, but it may work :rolleyes:.

Thank you very much. But the user have the option to resume and logging him off will also disconnect him from the current game server. When the program blocks the screen, the workstation should not be disconnected from the game server. any ideas please?

i found this example on freevbcode.com and knew you were looking for it, so just goto http://www.freevbcode.com/ShowCode.Asp?ID=508