So just last night I made the upgraded to 8.1 to find my mouse no longer works.
The mouse works on my laptop, as well as in the BIOS of the desktop in question, but not in safe mode.
In device manager it says "Microsoft USB Wheel Mouse Optical - Windows cannot load the device driver for this hardware. The driver may be corrupted or missing."
I've tried uninstalling the driver, plugging the mouse in to various USB ports, disabling & enabling USB controller settings in BIOS, uninstalling as many USB controllers in device manager before my keyboard stops working.
I'm out of ideas. At this point, is there anyway to go back to Windows 8 without losing my files?
The drivers for the Microsoft Wheel Mouse Optical are located here from Microsoft. Installing the Windows 8 driver should resolve your issue.
In regards to returning to Windows 8 without losing your files, you can use the Refresh and Restore feature of Windows 8 to return to the original installation environment without losing your personal data, but you may lose installed applications and need to reinstall them.