Assuming you're using WinForms, you could try handling the GotFocus and LostFocus events. Detecting whether a new program has been opened could cause issues, especially during startup, since you can't be sure your program is opened last, and Windows could be starting up a bunch of programs.
Is it also possible to stop it when I start up another programm ?
A particular program? Is it one that you have the source code for? You could always create a cmd file to start that particular program then create a shortcut to the cmd file and always start it from that. The cmd file could have a line to kill your program and another to start the requested program.
I have a scroling text that starts running at the bottom of my desktop when I start up my computer. I would like to know if it is possible to have it detect when any other program starts and have itself shutdown. Just curious if it can be done.