I think this one of those situations where we have to have some idea of what a good answer might look like, to decide if it's on topic. It's to that end that I shall discuss (but not answer) the question a bit, from a technical perspective.
I am aware of two good approaches to this problem (though I haven't used either and might not be able to write a good answer myself that explains them).
Use Windows installation media to reinstall the Windows boot loader. This is very simple. I see no harm in answering questions about how to do this, though providing advanced support for the rare cases where it doesn't work is probably off-topic for our site.
Boot from an Ubuntu live CD and install some boot loader that, unlike GRUB, is completely confined to the MBR and doesn't require that a
/boot directory exist anywhere to work. (That's why this error occurs when you remove the Ubuntu partition--GRUB needs to access data inside the Ubuntu partition to boot anything.)
To the best of my knowledge,
syslinux is the best (and possibly only good) choice for this. Information about how to install
syslinux to the MBR from an Ubuntu live CD is definitely on topic for Ask Ubuntu (even if the only operating system it will be booting is Windows).
I think a good answer can cover both of these options. Fundamentally, then:
- The question is about a condition created by Ubuntu but not by Windows.
- The solution that doesn't involve Ubuntu is very simple.
- There is an important solution that does use Ubuntu.
So, I think such questions are on-topic (though once we get a good, comprehensive answer to one of them, it can be considered a canonical question, and any others can get duped to it).