You've not told us which program it is which you want to remove but, in general, you can't uninstall a program which is actually running. Perhaps a component of the relevant program runs automatically when you start your computer, meaning that you'll need to terminate it (via the Task Manager) before trying to uninstall it. (Alternatively boot your computer into Safe Mode and try to uninstall from there, as most 'run in the background' programs aren't automatically launched in Safe Mode)