Hi all i've successfully flashed my ATV2 in windows. It took a while but finally got my head round it. Here's what i did:
You need a Mac or VM to create your custom firmware.
Follow the instructions here
to create your cfw and prepare your tethered boot files then copy your cfw and the tethered folder over to your windows desktop.
You also need ireb from here
copy the utility from here
into the tethered folder.
Once you've done all that follow the instructions below
1.Connect ATV to the PC via micro-usb and run ireb. Select Apple tv and follow the intructions to get into a pwned dfu mode (this is important otherwise itunes will error on you).
2.Open itunes and SHIFT-Restore. Select the custom firmware and wait till its done.
3. When its done disconnect the ATV from the pc and take it over to your TV and plug in the power.
4.Reconnect the ATV to the laptop.
5.Hold menu + down on the remote for 6 seconds
6.As soon as it turns off hold menu + play for 7-10 seconds to get to normal dfu mode.
7. Open CMD and run the below:
tetheredboot -i iBSS.k66ap.RELEASE.dfu -k kernelcache.release.k66
(you can create a bat file so that it automates this all for you by copying "tetheredboot -i iBSS.k66ap.RELEASE.dfu -k kernelcache.release.k66"
into a txt file and changing the file extension to .bat just make sure you put it in the tethered folder)
8.Once its done unplug the usb cable and plug in the HDMI DO NOT UNPLUG THE POWER CABLE!!!
If you ever need to tethered boot it again just run 7 + 8.