Can I get .apk file of an installed app?
It should be located at /data/app/.  You can access that directly if you have root.  If not, you will have to use adb pull.  For example:


  adb pull /data/app/myapp.apk C:\backup\myapp.apk


More info on adb: http://developer.android.com/guide/developing/tools/adb.html