Play to is a new function under Windows 7 which helps brings the ‘home network’ into, well, more of a home network for you and me. It allows you to play (stream) media from one computer to another computer on your local network. Media could be music, videos or recorded TV.
It is based on streaming – and is smart enough to adjust itself to the network and resources of the machine you are playing the media to. It is not based on file sharing of the local network, and as such, you do not need to setup and file sharing between the machines. The actual file does not get transferred to the remote machine, so the remote machine cannot record or copy the media without third party programs or hardware. This function seems to have been created so it will not make groups such as the RIAA upset at Microsoft, but still easy and featured enough to be useful.
It is great because it works straight out of the box with minimal configuration. You do not need to install any additional software. As far as I can see, it should work on any version of Windows 7 (from home versions to ultimate etc). You do not need to have the fastest network, a simple wireless card running at 11mbit/s will be fine. Of course, you this will work great for audio but for video – you would want something a little faster.
From what I can see, there are xxx requirements to make “Play to” work under Windows 7
First – you need to be in the ‘home network’ group for networking. This means, if you are in a domain or public network – you will not be able to stream media from one device to another.

(picture of what the home network group looks like under networking and sharing)
Second – you need to have “Allow remote control of my player’ enabled under Stream of Media Player 11.

(the steps you need to perform under Media Player of the remote machine to enable streaming/play to)
Third – you need to have media player open on the remote computer. Please note – Windows Media Centre is not Media Player, and as such, if you have Media Centre open – the play to function will not work unless Media Player is open too (in the background).
Other then the above – dead simple. Once you have ‘allow remote control of my player’ enabled – from almost any media on your local machine, you can right click it and have a ‘play to’ menu which allows you to select the remote device. You can do this from inside a library, such as inside Media Player, a folder or a file. From my testing with single audio file – the remote computer started to play the song within a second. A video, 500mb uncompressed AVI started in 3 seconds.

(how you would play some media by right clicking on a file)

(this is what the host (ie, the machine you had the media on) will show - you can skip, pause, control audio levels etc)
There are still a few limits in the play to system. Probably the two most annoying at the moment is when I close Media Player on my machine, the remote machine can sometimes take 10 seconds to respond and stop playing the media. The second and most annoying missing feature is random. If I have a folder of 200 songs playing – there is repat but no random which means it will start at song one, and continue until song 200.
But otherwise, this is a nice addition to Windows 7. I would love to see, or hear if they have addressed the random function in a later or RTM build.