I'm a complete noob when it comes to jaibreaking devices, but I am familiar with Linux command lines, etc. and would like to install XBMC on my AppleTV, so I can stream other video files.

The AppleTV2 device which I purchased, has already been upgraded to Apple TV 5.0 (latest icon driven UI) - note that I haven't installed the very latest update.

Is it possible to do an untethered jailbreak of this device and if so, can anyone point me to an appropriate web page?

I've read some conflicting information about this, but I think it's not possible, without downgrading the firmware on my AppleTV2. If that is the case, where can I obtain instructions and proper versions of the relevant firmware?

There's a lot of information out there and I haven't yet found one definitive resource which I trust!
I would be using a Windows PC in order to do any jailbreak.
Thanks in advance!