Microsoft has made freely available a pre-release beta of Windows Thin PC, a limited version of Windows that is meant to provide basic modern operating system features for older systems, and repurpose them as thin-clients.
Called WinTPC for short, the version available for free is the Windows Thin PC Community Technology Preview test build – the final product however, will be meant only for Software Assurance volume licensees.

readme.txt
Activation
==========
This evaluation copy will be automatically activated if the product is connected to activation server in the first 3 days after installation.

Evaluation period for fully activated copy of the product is 90 days.

If not activated in the first 3 days, default evaluation period is 30 days.

There is a known issue with activation of this product. It is not possible to activate this build using "Activate Now" option. However, you can activate using command prompt by issuing the following command:

slmgr /ato

What are the biggest differences between this and a 'normal' copy of Windows?

Since WinTPC essentially is a thin client solution, here are some key differences between thin clients, zero clients and PCs:

* Hardware Configuration: Traditional thin clients differ from a PC in that they have a lower CPU and less RAM than a typical PC, typically use less energy and have very little (if any) local storage for data and applications.
* Zero Clients: New form factors such as Zero Clients are also piquing interest in the market. They are thin client tierminals with no local operating system or storage, and depends on the server for all of its computing and translation capabilities..
* Functionality: Traditional and Zero client devices must always be network connected, cannot run local applications. They are essentially single purpose devices.
* Management: While thin clients require less management than a PC, they are not “zero management” since they also require firmware and security updates. It’s important to choose a thin client that can integrate with our existing management strategy and tools.
* Cost: Although traditional thin clients reduce management and operational costs, they are not free since there is an upfront acquisition cost. Depending on the device and the capability, a thin client could cost as much as a low end PC. Many of you told us that budgets for buying new devices have been reduced, and that you prefer allocating funds towards devices that offer more functionality and flexibility, such as new Windows 7 PCs, tablets, or slates. Additionally, all traditional thin clients and zero clients require VDA licensing for VDI
* Multimedia capabilities: Many VDI vendors use the graphical processing power of the end point device to render multimedia for the remote desktop in an attempt to improve the user experience. Not all thin clients or zero clients are able to execute high fidelity graphics locally, and those with built in multimedia capability may cost more than those without this functionality. Our guidance is to evaluate the costs of these devices and weigh them against your business requirement to ensure you choose the right one.