In this tutorial, I will use Windows 7. Vista will be similar, and you should be able to work the differences in XP. A Mac Tutorial is available at this post
Your end result should be like this...
~Go to Control Panel~
~Once in Control Panel... Go to 'Clock, Language and Region'~
~Click 'Change the Date, Time or Number format'~
~Change your Time Formats so that they show AM and PM~
Sorry the picture isn't very clear... You need to change the HH:mm and the hh:mm:ss so they have tt after them.. EG hh:mm:ss tt
~Click on Additional Settings~
~Click the Time tab~
~Change AM and PM to your own message~
There is a limit to the length, but Sharenator fits, so lets use that for our example.
TIP: Make AM and PM the same message for effect
~Hit apply, and look at your clock!~
That's it! Note that this does change your clock's AM/PM to Sharenator, so on emails etc, it may say: 'Recieved on 12:43 Sharenator'. Have Fun!