You downloaded via the browser (http/ftp) ? I'm not sure what the proper md5sum is for Shift, to make sure you got
exactly the right data without corruption. I think torrents are verified as part of torrent process, might want to try that method.
Once you have a verified a good download, you don't need to guess and try re-downloading. It is good, and you can proceed to the next step, the burn.

Burn the verified iso image to a CD. Then verify the CD is an exact burn. Most burning software includes this ability. Linux also allows just doing an md5sum on the whole drive, but you might have to find a utility for Windows that can give you an md5sum of the burned CD.
Make sure that the image is burned accurately. You will save headaches and CDs this way.
Then it is all about does Shift boot, or does it not.