my account has been hijacked

Tell them what your gametag was. They will ask any other questions they need to know. I am pretty sure the old tag would be enough for them to start a case on this. Let me just ask it this way. Is it better to at least try and maybe fail then not to call
at all and let them get away with it??

It tells you to have your Gamertag when you call though and this isn't my Gamertag!

Have Just had the exact same thing happen, recieved an email at 11:01 saying I had bought two sets of 5,000 XBL points and after having reset my window's live password (as it had changed!? 14:15) I logged in to find someone had added achievements to Fifa
12, a game I have never owned and I cant speak to a real person Via a telephone.

my billing states 22/10/2011 PREMIUM GOLD PACK -- Game Consumable -120Microsoft Points 10Microsoft Points

for 10,000 points worth

Thanks, you're right. I just wanted to be sure before I even try.

Phone Xbox support ASAP
support.xbox.com/.../contact-us