I was successful twice with WIN7 and DPU mode for my Apple TV 2, and jailbreak worked both times. Basically I just ignored the USB device not identified. I did move it from USB connector to other connector, as I have about eight different USB outlets on my computer. try moving it around to trick WIN7 into finding it again.
This is a pain but you will eventually have success. Get GREENPOISEN started, and plug in your Apple TV 2, (no power just data). Then immediately hold down the buttons on your remote to put the Apple TV 2 into DFU mode. Eventually it will work. Also you can try adding power cable , but no power seems to work better.