[whysoserious]

@"sixpack58" 

Set your HTPC to boot from USB as the primary choice. Then...

1. Bootup LibreELEC USB thumbdrive created with the  “LibreELEC USB-SD Creator” app from the LibreELEC website (or make it with Rufus or your preferred software tool).
2. After booting you am presented with a menu with 3 choices: Install, Run, or Live. 
- Install installs LE on the hardrive of the HTPC, wiping out any existing OS's and files.
- Run installs LE onto the same USB thumbdrive in a "persistent" state, and saves changes such as libraries and add-ons. Additionally, it makes the USB thumbdrive boot to the "Run" condition by default from then on. On a USB 3.0 device in a USB 3.0 port it is as fast as a native installation.
- Live runs LE from the USB thumbdrive but I don't think it is persistent, doesn't save any changes you make, libraries, add-ons, etc.

So type Run and hit Enter before the Installation timer runs out.

Wait for LE to do it's thing, your HTPC will reboot into LE, running from the USB thumbdrive.

This method uses the HTPC's ram and hardware, but doesn't make any changes to the device's hard drive.