Hi
I will try to offer some limited help out of my memory, I am not at my QNAP right now.
I have my music, photos and some videos in the Multimedia folder. Nothing has ever happenet to them - QNAP has never tampered them in any way - so I consider it safe and harmless to put them there. But no need to, if you use some other apps, DLNA software etc to serve them.
QNAP is a computer, and it has its own users and access rights to those users. The users are set up using the admin user in the QNAP software, and the access rights are also set there. You will need to learn the basics of the QNAP software, but it is easy and straightforward.
Then you need to map network drive in Win8 and use the username/password you have in QNAP to logon to that drive in Win PC.
When you set the QNAP users and access rights so that only users with password can logon to QNAP to access the system and the files, you should be all set