According to Microsoft there are three different Alt+Numpad sequences offered by Windows (news to me -- I always thought there was only one).
Alt+Numpad: A method of entering characters by typing in the character’s decimal code with the Numeric Pad keys (Num Lock turned on). In Windows:
* Alt+<xxx>, where xxx is the decimal value of a code point, generates an OEM-encoded character.
* Alt+<0xxx>, where xxx is the decimal value of a code point, generates a Windows-encoded character.
* Alt+<+>+<xxxx>, where xxxx is the hexadecimal Unicode code point, generates a Unicode-encoded (UTF-16) character.
The following article attempts to actually describe their usage:http://www.irongeek.com/alt-numpad-ascii-key-combos-and-chart.html