The files Lightroom leaves on the desktop should be the install files that were downloaded. You can delete those after your install is good (or keep the latest in case you need it).

As an IT guy, I would be hesitant to mess with files in the windows directory. Just remove older versions if they are still installed via the control panel (which it sounds like you only have the latest).

If you have an SSD and a traditional internal harddrive, you can move your backups to there.

I also try to keep very little on the computer harddrives. I put as much of my data (including the photos) on an external network drive. It contains 2 mirrored drives. That way I can easily access it from all the computers on the network as well as make it easier to change computers.

Eric