Here's the deal. When you run your sofmod installer, you usually have an option to backup your original M$ dash... do it. If for some reason a game attempts to update the dash and you receive error messages OR you have problems like the ones you're having, then all you would have to do is boot up the softmod installer stuff again, and select the Restore M$ dash option.
Since it sounds like you didn't backup your m$ dash (c: drive) to either your xbox or your PC, you will need Slayer's or Auto Installer Deluxe. Both of them have the newest M$ dash (the files you need).