I got it to work for me using "XML: \windows\regist_ry.xml" . I never tried the extionsion .provxml. You can also name the file something like "mxip_MySecurity_Off_100.provxml", cook it into your rom and windows should run it on start up.
Of course keep in mind, one little syntax error in your xml code and the whole xml/provxml file is bad and won't execute anything. For that, I nominate Boggsie!

You might want to share and post the file, we can check it for you...