We've seen a few users reporting warnings from Windows Defender when downloading Cura 2.6.63. This appears to be false positive as users have reported the warning does not appear for future downloads. See https://forum.lulzbot.com/viewtopic.php?f=8&t=7044 for more info.
Two additional users so far reported the issue (from helpdesk). First user reported on Windows 10, Cura 2.6.63, he tried two different machines. Second reported the same error from the forum, Trojan:Win32/Fuerboos.A!cl
Report from second user, "I had to temporarily disable Windows Defender Real Time protection so that the file wasn’t deleted before I could run the installer – then I had to tell Windows Defender to ‘allow on device’"
Downloading a second time did not seem to work.
@lansky If this is run into again, assigning this as an exception should get them up and running: https://support.microsoft.com/en-us/help/4028485/windows-10-add-an-exclusion-to-windows-defender-antivirus