after seeing some .reg files in my registry i made some thing like this
with that i was able to open my .abc with mplayer by default 
my questions are:
1.Is it right or need to change the code
2.what if i need to associate the app with .dat,.vob,.mp4 .....etc create a reg file for all the types
well just
this line Windows Registry Editor Version 5.00
is duplicated remove the second one
and to make it working for types like dat vob etc
[HKEY_CLASSES_ROOT\.abc]
@="Mplayer"
[HKEY_CLASSES_ROOT\Mplayer]
@="abcfile type"
you need to make this for each one of them
Windows Registry Editor Version 5.00
[HKEY_CLASSES_ROOT\.abc]
@="Mplayer"
[HKEY_CLASSES_ROOT\Mplayer]
@="abcfile type"
[HKEY_CLASSES_ROOT\Mplayer\shell\open\command]
@="\"C:\\Program Files\\Mplayer\\MPlayer.exe\""
you will only make the first two section for every type
and in the third one the destination for MPlayer.exe
you can change it with this
[HKEY_CLASSES_ROOT\Mplayer\shell\open\command]
@="\"%ProgramFiles%\\Mplayer\\MPlayer.exe\""
so it works with any program files