If you want something like KOrganizer for reminders, you can install Evolution Mail and Calendar from the Software Center. I've tested it and it works in Ubuntu 11.10.
You just export from KOrganizer to .ics format and import it in Evolution.
I saw Evolution Mail and Calendar available for Windows too. Just Google it (Note: I've never tried this yet in Windows).
This is good for Windows 7 users since there is no default Calendar reminders for it, unless you buy Microsoft Office with Outlook in it, which is really expensive nowadays. I really can't understand why they removed this feature that is available in Windows Vista.
I guess, we'll never know.