Yep, It does.when you install a 3rd party AV it automatically deactivates Windows Defender from services (at least Avast does).
Unlike Windows 7, which never had an Antivirus Product, Windows 8 has it integrated in the system. Installing an aftermarket AV product would only disable the service, but somehow will always try to run the first time you run your computer (it happens occasionally so its not big of an issue)
When we already have a brilliant product pre-installed, why would we choose a separate one for that anyway? Thats my only concern.
Even if you have to use a separate one, then wouldn't it be better to atleast download the compatible (preferably Latest) version instead of trying to install some old version? (Thats a separate thing that most of the good brands like Norton, BitDefender, Kyspersky, would run just fine even if you try to install the Win7 version on Win8 -- But its better to download the latest version)