An easy way to keep the program from appearing in the Alt+Tab list is to set the Form’s FormBorderStyle property to be a ToolWindow (fixed or sizable).
One caveat to this is that if you display another Form (that is not a ToolWindow) through a call in your application, the main Form will again appear in the Alt+Tab listing. To get around this, you can make all of the Forms in your Tray application have a FormBorderStyle of ToolWindow.