Are you running the latest .net framework? Also, is your windows on the default aero theme? I can't run Horizon on custom themes because of a framework error. You could also use Le Fluffie for r&r but i'm not sure if it's kept up to date anymore.
I just got Diablo to work today. Been trying for days. May have temporarily misplaced my original save... But. Same issue. Eventually I used 360rev, redid the console id and whatnot to match mine, rehash, resave, went to Horizon, injected to hd in the correct folder since it wasn't working on 360rev. And magically it worked! Now I just need my original saves back. Just keep trying slight variations of what you're doing.