First thing to do is to make sure that you have all the latest McAfee and Microsoft security updates. These things often get onto a system because the security information hasn't been kept up to date.
Second, run a full McAfee scan. If McAfee can detect and fix this then a quick scan might be enough, but it does no harm to run a full scan from time to time anyway. If McAfee finds anything it will quarantine it, giving you the option of deleting, keeping or sending to McAfee for analysis. Send anything that the scan finds to McAfee, and if it's new it'll be added to the threats database.
If McAfee finds nothing (and some of these slip under the radar) then download and run Malwarebytes or SuperAntiSpyware. They're both free; in each case you only need the stand-alone on-demand scanner, not the real-time always-on system (both of which will clash with McAfee). Run either, or both; SuperAntiSpyware may pick up things that Malwarebytes doesn't.
Oh, and I've moved the discussion from the Home & Home Office section to Security Awareness, to join the other posts about fake AV's.
For some reason this thread is returned high in the google search when looking for "defender.exe" so I'm going to post up some advice and then lock the thread.
If you need assistance with a new undetected version of a fakealert infection please start a new thread in our Top Threats space, however first of all do try and remove the infection using our FakeAlert Stinger tool - instructions for which can be found on the link.
If you would like to send us a new variant of a defender.exe please follow these instructions. Once you have submitted please post up the analysis ID we respond with in the
Top Threats space so that we can follow up on it for you.
Hope this helps,