In watching your 3 videos:
1. Disconnect your computer from the internet. Or at a minimum, block access to TCP ports 445 and 139 in the/a firewall.
2. Disable your computers AUTORUN "feature' on your removable drives(ie Garmin)
3. Re-attempt running RMPREUSB
On the chance that the above allows you to to properly flash the nuvi, do not plug the nuvi back in until you have updated the operating system(Windows Update) and the virus signature files of your anti-virus software _and_ completed a FULL system scan of your computer.
You say your computer is clean and virus free. I say it is not. Watching the videos proves me right, you have an infected computer.