The scenario you describe essentially amounts to the use of each workstation as a thin client to access a centrally located desktop environment. It would be highly impractical for Window 7 to boot from PXE even if it could be done. Whenever PXE is used to boot, it downloads the entirety of the boot image to the client system, which would mean several GB of transfer at each boot.
Ideally, this scenario is accomplished by keeping the desktop environments on the network in the central location. In a Virtual Desktop Infrastructure (VDI) environment, this is accomplished using virtualization to allow separate virtual desktop environments to reside together on hardware, the virtual environments are provided to the clients through a manager. In a session based environment, each user’s desktop environment launches natively on the Server and is brokered to the clients through a technology like Remote Desktop Services.
In both instances, the workstation must still run an operating system; however it is typically a very lightweight operating system providing a basic interface for the hardware and a client for redirection to the server hosting the desktop environments. For customers with Software Assurance, Microsoft provides Windows Thin PC as a lightweight operating system designed to connect through Remote Desktop Services to a Windows Server. Additional features supported like RemoteFX support for enhanced graphics, DirectAccess VPN connectivity, and BitLocker encryption help to provide the optimum thin client operating system for repurposing desktop hardware.
If the above sounds like the right route for you, you can find more information, guides for IT professionals, access to trials and betas, and much more in the Desktop Virtualization Center of the Springboard Site on TechNet.
Windows Outreach Team- IT Pro