This trick works on Windows XP, Windows vista 32/64 bit and Windows 7 32/64 Bits.
So to be able to do this, you must:
Go to the Windows registry : Type Regedit program through the "Run" in the "Start" menu.
Then expand successively the keys :"HKEY_CURRENT_USER"/"Control Panel"/"International".
Double-click the key "s1159" and type the desired word in the "Value data" field. Make sure that the word begins with a space, otherwise your word will be glued to the clock of Windows, and will therefore be difficult to read. Entire word is limited to eight characters (including the empty space).
Double-click on the keyword "s2359" and type in exactly the same word, including the empty space.
Finally, double-click on the key "sTimeFormat" then replace the contents "HH:mm:ss" by "HH:mm tt".