I've done it before. I wont say it's easy, but it is doable, and it taught me a lot
You'll have a hard time if you want it to be 2003&&Xp. That is not how diskless thin clients work.
The most important bit is to decide exactly what you want and how you want it.
Do you truly want a diskless thin client? Or wouldn't you mind if some of the OS is stored locally? Do you want to use PXE boot to push an image from the server to the clients, or do you want to use nfsroot and pull the filesystem off the serve from the clients?
I've never heard of a .tec file. But that's no matter. Many providers of thin clients provide their own file format. 64m or whatever. They are often just .tar.gz with additional pre-defined scripts and binaries inside them.
Can you be more precise about what it is that you want?
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Can't tell an OS by it's GUI
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