The easiest way might be to open Regedit, navigate to the
HKEY_LOCAL_MACHINE\SYSTEM\MountedDevices folder, make sure the MountedDevices folder is highlighted on the left, then do File->Export and save the registry keys somewhere. It should save as a .reg file. Upon reinstallation, just double click on the file and it should import them back to restore the settings.
Make SURE after you export the keys to verify that they were all saved and reflect what is currently in the registry by right clicking on the saved .reg file and opening in notepad/wordpad.
I believe this should work.
For reference, check out Option Two below:
Drive Letter - Add, Change, or Remove in Windows 7