Hhmmmm.......................
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I'm currently using Webroot Secure Anywhere for Gamers anti-virus software w\ my box in concert w\ Windows Defender enabled as well to periodically scan for threats in a limited state but Webroot is set up to do the heavy lifting (this version of Webroot does play well w\ Windows Defender).
Just downloaded the latest version of AHIII & all went w\o a hitch......didn't have to disable Webroot or Windows Defender either & AHIII isn't set up to be excluded from either software as both are actively operating.............
Webroot does make several versions of anti-virus software depending on computer usage so it may be beneficial to find out which version the OP is actually using.......why I posted that I'm using the Gamer's version of Webroot Secure Anywhere AV as this version was specifically written to coincide w\ computers running game software (think online competition gaming) but still provide good AV protection.
Haven't had any issues since installing this AV software on 2-'17 after dumping Kapersky (major resource hog to go w\ the false positives).
Will keep on the lookout for any nefarious activity going forward but so far I have had no issues out of this version of Webroot.
FYI....................
