1)Open control panel -> region and language
2)Click on formats.
3)Click on additional setting.
4)Click on time.
5)Change the AM or the PM symbols to the name you want.
6)Then press apply or OK.
7)The trick is done.
Its a funny trick that can be easily done and you can surprise your friends with.
The steps i gave is for windows 7 ,but it can be done on windows XP and vista but their will be a slight variation in the steps.