How do I safely install an app not from the Play store? I have always installed apps from the official store but today I wanted to install an app from SlideMe so I clicked "download" but it didn't install automatically.  It just downloaded an .apk file and then a window popped up with two options called "Google installer"  and "package installer"..how do I proceed?  I have an unrooted S3.

OS: 4.1.1
Choosing the Google installer option will verify the application with Google before installing. Choosing the Package installer option will install the application as is. Since I like to be safe, I installed AVG Antivirus from the Play Store. A third option will be to scan the package for viruses before installing.