System.Windows.Forms.Timer will execute in the UI thread
System.Timers.Timer executes in a thread-pool thread unless you specify a SynchronizingObject
System.Threading.Timer executes its callback in a thread-pool thread
Feel free to comment if you can add help to this page or point out issues and solutions you have found. I do not provide support on this site, if you need help with a problem head over to stack overflow.