redirect to latest version:
http://status.calibre-ebook.com/dist/win64
index of past and present calibre versions:
http://download.calibre-ebook.com/
If this link is downloading malware, you have a pre-existing infection that is interrupting your download.
If that is not the link you downloaded calibre from, you have clicked on an ad that you shouldn't have...
Millions of people have downloaded calibre without getting malware, I feel safe assuming Kovid isn't distributing malicious software.