.htaccess is not working os win XP and server apache

[php4om]

Hi there

I have tried to make a custom error 404 page, using .htaccess

It works as planned when asking for a file / page that is not there, except if the request is for a .php file, then the server dont load my custom error page, but shows the standard 404 error message

How can I solve that?
Do I need to make some kind of php script or what????

Thanks in advance
om