I am having the exact same issues. My work received a Dell Latitude ST tablet with Windows 7 on it. After trying to install Windows 8 x86 from both a DVD (created using the Windows 7 USB DVD tool) I get the original driver error. This is using an external DVD rom since the tablet doesn't have one built in obviously. I have re-downloaded the file multiple times and I still get the same error.
I used a 16GB flash drive and created a bootable image using the Windows 7 USB DVD tool and I still get the load driver error. I have downloaded all drivers from Dell's website and placed them on the thumb drive, then browsed to each folder one at a time to see if any of the drivers will work. Apparently the driver it wants isn't something Dell offers.
At this stage, after screwing with this thing for 6 hours yesterday we are prepared to just send it back to Dell and swear off Windows tablets all together. It's a shame since we are primarily a "Windows shop."