Does it exist? Is it Linux only? Do I have to install it?
According to this, it's
:!xxd to convert the file to hex, and it's
:!xxd -r to convert it back again.
It is distributed with Vim (as xxd.exe), along with diff.exe.
So it exists and should work on Windows. You don't need to install anything separately, as it is already bundled with Vim (at least in the versions available from vim.org).