Hey dude.
It seems you are one of those accounts used to sell those 10.000 mspoints accounts for 25 bucks on ebay or something.
AFAIK, they didn't do it through your xbox, they did it through Virus on your machine. Through Games for Windows Live, they can figure out your password (or if you ever logged on the XBox Marketplace through your web browser) and then purchase the items and transfer to another account, which they will then sell for people to redeem and use the points.
They even say things like "these points will last only for 24 hours, so use the points quickly, as the games won't be deleted".
Call Microsoft.