windows installer - device driver not found

[REDDICE0818]

I want to install Windows 8.1 Enterprise Evaluation on my PC. The problem is that either the .iso can't be found or the device drivers can't be found. After downloading xxxxxxxxxxxxx.ISO.part I decide to try to mount it to my USB drive via Windows 7 USB/DVD Downloader Tool. No matter what I do I can't get the tool to find the file I downloaded. I managed to trick it but tells me that it's not a valid .iso. I then decide to use another program called Rufus. It does manage to mount the 3gb file into my USB drive. After getting my PC to boot into the USB, I end up at an installation. Just as I think that all my problems are solved, I'm told that that the device drive with media can't be found and to insert the CD/USB. Is there any way to fix this?

[Semper]

if the file is ending in a .part, that means it's incomplete so far as I know.