I just noticed this service is running despite me having had disabled Windows Defender and all of its startup entries and disabling the Windows Security notification icon. Is it safe to disable this service if I am using a 3rd party AV (using Avast Antivirus Pro)?
I have Defender removed from my install, and Windows Security Service is not present, so I would say it is safe to do so. Caveat: I am not running any other AV, so there could be an issue.
It is perfectly safe to disable the service, i have it disabled on all of my machines and i use newest version of ESET Internet Security x64 as my AV. @boyonthebus The Start Menu icon will be gone if you remove SecHealthUI from the offline install.wim image via provisioned registry key, you can also use Optimize-Offline script to do that, i've done it with NTLite myself, i don't think it's possible to remove it from a live system.