Associate file extension with program

Hi,
I guess this is an easy one for most of you.

I could not find out, how to associate a file type by its extension with a program, so that on double-clicking the file, the respective program opens with the file as input.
I know how to do this on the desktop, but have to do it programatically. In the registry there are certain keys to establish an association, but it seems to me quite a hack to set an association just via these keys.
Regards,
Robert

hi,
I learnd about associate file extension with application from Lee Weiner.
This is his URL : http://home.fuse.net/lweiner/
go there and download the associat.zip file.

hope this will help. :)