imagex is an official Microsoft program. It does not come with Windows. It comes with what's known as the
WAIK (Windows Automated Installation Kit), and requires you to download a gigantic EXE or ISO to deal with, blah blah blah -- the usual Microsoft rigmarole (waste tons of bandwidth and disk space and installing a bunch of useless nonsense just to get a single file).
If you're looking for the imagex.exe binary, I'm sure you can find it, or ask someone (hint hint) if they can send it to you. It's a standalone executable. There are 64-bit and 32-bit versions of this binary, and which one you use depends on which OS you're running (NOT which OS / .WIM file you are installing).
Please be aware that you can completely destroy the contents of a filesystem using imagex, so if you pass it the wrong drive letter, you could be destroying the wrong thing.
It is not a partitioning tool -- it simply overwrites the contents of a filesystem with whatever .WIM file you give it.
You may also need to use bcdboot.exe to reinstall the boot blocks and bootloader (depends on the behaviour you get after doing imagex + rebooting). Like imagex, there are 64-bit and 32-bit versions, and which one you use depends on which OS you're running.