Wow. I just googled and came across a thousand results, it's a DNS problem on your end, go call your ISP about it. Also some say it can also be the family filter on so set that it to accept all content
I've fixed it!
I still can't download from my Xbox 360, but I've found a solution.
I signed up for another Windows Live ID, and used that to sign up for an Xbox live account, stating that I'm 18 only so I can download from http://www.xbox.com's marketplace.
It should work, because I can still download things through Bungie's File Share and things like that.