How to?-Prevent multiple instance of an application

One of the best and easiest way to prevent ur app. from running again when its run , is to create a named Mutex for ur app. then you should check it at the begining of ur app. and if CreateMutex returns the ERROR_ALREADY_EXISTS , then you can be sure that another instance of ur app is running and do whtevere u want ! (Find more info about the "CreateMutex" API in MSDN)