Preliminary scan results show that malicious or unwanted software may exist on your computer.

When I scan with Defender, a message at the bottom reads "Preliminary scan results show that malicious or potentially unwanted software may exist on your computer.  You can review detected items when the scan has completed."  I always get a message when finished that says no threats detected.  Why does this keep happening?  I am afraid to do anything with my banking etc...I have all the current updates and I have ran a full scan and restarted my pc.  I still keep getting this message about malicious software every time I do a scan.  Help!  This all started on 11/28/16.

That means when the scan was finished the Windows Defender program determined that the software in question was not malware.

If no items were detected, I'd just ignore it.

Or

You can always scan with one or more of the following (free) independent scanners.

Emsisoft Emergency Kit: http://blog.emsisoft.com/2015/06/09/how-to-find-and-clean-malware-infections-with-emsisoft-emergency-kit/

Malwarebytes Free https://www.malwarebytes.org/free/

But if you have concerns or issues with the Windows Defender program you may want to install a third party program (which should disable Windows Defender). 

There are a number of viable options available – free or paid - take your pick…

Choosing an Anti-Virus Program

Is Windows Defender The Best For Windows 10?  Good article but ignore the ads.

http://windows.microsoft.com/en-US/windows/antivirus-partners#AVtabs=win10

Regards...

Tips To Protect Your PC

https://malwaretips.com/blogs/how-to-easily-avoid-pc-infections/

http://blog.emsisoft.com/2015/01/27/top-10-ways-pups-sneak-onto-your-computer-and-how-to-avoid-them/