One really easy way to do this is to make a script, and just have it run at startup. It requires no programming skill, just Notepad.
Make a new text file and name it whatever.VBS. Now put the following text in it:
^^^^^^^^^^^^^^^^^^^^^^^^^
Dim WSHShell
Set WSHShell = WScript.CreateObject("WScript.Shell")
WSHShell.Popup "This is a message"
^^^^^^^^^^^^^^^^^^^^^^^^^
That will pop up a box on startup with the text that's in quotation marks.