I've got a Surface Book, purchased at a Microsoft Store, so I could be sure to have a "clean" Windows installation. Well, yeah, it's clean, but the Windows Defender AV is still annoying. Whenever I'd try to upload something from my "Downloads" directory, the file picker dialog with hang forever with a "Working on it..." message rather than a list of my files. After a minute, it would then work.

After a few weeks, it decided that the installer for IntelliJ (2016.2.4) was a malware installer and quarantined it, and then everything worked great thereafter. I'll chalk that up as a false positive. (I hope.)