If you're making a back-up of a game that has already been backed up to a disc before, you can just use DVD decrypter to back up the game to your PC as an iso and then burn it to disc using ImgBurn or DVD Decrypter. However, if you're trying to back up an original game (retail version), then you'll need to use an Xbox and FTP to do it.
That's normal... a PC can't read an Xbox game, regardless of whether or not it's an original. If you have a back-up copy already, just have DVD Decrypter make an ISO. You won't be able to browse it, but if you burn it to disc it'll work fine.