Correction... Most applications can be installed and run just by installing the APK file. As for Google Now, you do need to be rooted because it has to write to /system to work.
Well I found some zip files from XDA with a one click install. It mentions backing up stock Google Search or renaming it but I can't find the files on my device.
You still need to be rooted to use google now. You cannot flash that without a custom recovery, and to get that you need an unlocked bootloader (and most likely root access)