You can roll back to 12.0 without impact on your library (there were no database changes between 12.0 and 12.1) - see Turingtest2's notes on Troubleshooting issues with iTunes for Windows updates for steps needed to completely remove all installed iTunes components, and for links to the 12.0 installers.
To go back to a previous release, you'll need to restore the iTunes database that was valid prior to the upgrade to 12.0. You'll find this in the Previous iTunes Libraries folder - files will be named iTunes Library yyyy-mm-dd.itl, where yyyy-mm-dd is the date when the relevant update was installed. You can (with iTunes not running):
- rename the current database file - normally found in C:\Users\username\Music\iTunes - from iTunes Library.itl to iTunes LibraryXXX.itl or similar.
- copy the relevant iTunes Library yyyy-mm-dd.itl file from Previous iTunes Library to the folder where your active library is located (the file you just renamed)
- rename the iTunes Library yyyy-mm-dd.itl file you just copied to iTunes Library.itl
- download and reinstall the correct version of iTunes
As long as the library database you just copied is compatible with the legacy version just installed, you won't get the "cannot be read ..." message.
Note, however, that since you're restoring a library database that is several months old (e.g., if you're rolling back to iTunes 11.4 using the previous library file created when you installed 12.0 this could be as old as mid-October) any additions to your library, changes to playlists, etc. that you've done since then won't be present in the restored library.
