Open regedit
Navigate to
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Class\{4D36E980-E325-11CE-BFC1-08002BE10318}
You should see 0000, 0001 etc keys.
Delete all of the 0000, 0001 etc but leave the
{4D36E980-E325-11CE-BFC1-08002BE10318} key and the properties intact.
Close regedit and open Device Manager.
Unistall the Floppy devices.
Reboot. The system will now redetect the floppy devices as though it was the
first time at setup and your default drive letters should be restored.
These are static keys that will retain their settings to reflect any option
changes a user makes hence the reason why simply uninstalling and
reinstalling from device manager does not work.