well i aint sure if this is the best way to fix it but what i would do is download "XBoxHDM" and use that along with Kingroach's NDURE installer to fix and resoftmod the xbox.... u will probably need a freshcopy of msdash to.
basically the XBoxHDM program runs from a bootable cd on your pc and then u hook up the xbox's hard drive into your pc and then u can replace the msdash files on your C partition and then resoftmod it ... once u got it all setup correctly it's fairly simple to use.
WARNING: supposedly it's possible to damage the xbox/pc using this method although i did it several times and never had a problem... supposedly the danger comes from "hotswaping" the IDE cables when the power is on.
basically if u dont know what u doing i would probably NOT do this! , but me personally i did it atleast 4times or more and it's always worked for me although someone else i talk to online had some problems using this method.