In this question posted yesterday, the OP has a Wubi system that will not start, because the Windows boot loader (which loads Wubi--in a Wubi system you select Ubuntu from the Windows boot menu) has a problem loading
The OP was trying to boot Ubuntu, not Windows. S/he was telling us the truth.
wubildr.mbr is Wubi's boot record. It is not present on a Windows system that doesn't have (or hasn't had) Wubi. Not ever.
An error relating to
wubildr.mbr is an error about Wubi (and thus Ubuntu). There is never an error about
wubildr.mbr when the Windows boot loader tries to boot Windows, because
wubildr.mbr is not Windows.
Unlike Ubuntu's boot loader (GRUB), the Windows boot loader is not designed with the assumption that it will regularly be booting non-Windows operating systems. Like most software, the Windows boot loader has error messages written to capture the most likely causes of errors.
So the error message said:
Windows failed to start.
But the OS that failed to start here was not Windows.
This question is an on-topic for AU as any Wubi-specific question. Does this have to do with Windows? Yes, it's an error given out by the Windows boot loader, informing the user of what is most likely a broken Wubi installation. I cannot think of any other Wubi-specific question that is more on-topic.
We should reopen this now.
A Lesson for the Future
Anyone could have mistakenly closed this. I could have. For all I know, perhaps I have made this very mistake myself. And that is all the more reason we should try to learn from this.
Here, the OP said they were trying to do one thing. Microsoft Windows said they were trying to do something else.
The mistake here was to trust a Microsoft Windows error message over the OP's pretty clear, detailed description of what they were trying to do and what happened. We assumed the OP was not telling the truth when s/he claimed to be trying to boot Wubi. We assumed it because Windows said so.
I think that pretty much sums up the matter, but if anyone has other thoughts (especially disagreement), please feel free to post an answer!