As far as I know (somebody can correct me if Im wrong), once the tamper sticker (silver sticker, under faceplate) is removed, MS fixes it for a flat fee ($150 i thought). I have fixed several I bought off E-bay, and I'm guessing yours is done from what you are mentioning. I know I sent a couple i got of E-bay in (a year ago or more) without doing anything (regestering or anything, just called MS) and got warrenty so I dont think you have to have regestered it with MS. But if its been opened at all and MS knows, you are screwed.
My advice, sell it on E-Bay, keep your HD (if you have one) and pick up the new arcade pack. You should be out maybe $50-100 max when all is said and done.