You don't see what difference it makes?! An MD5 digest is a "fingerprint" of a file, basically it's a simple way to represent the contents of a file to know if two files are the same. The MD5 digest of your file is different from what it should be, meaning that your mirc.exe is not the same as everyone else's. The fact that nobody else is having this problem is just further proof that this is the cause of your problem.
As for why yours is different, I have no way of knowing. It's possible that the installer you downloaded from this site got corrupted. It's possible you've got a virus that is changing the exe file. Or any number of other things. The point is, this is the source of your problem.
