Windows 7 allows users to display multiple clocks (for different time zones). Is there a way to have multiple calendars there too? For example, can I display both Gregorian and Islamic (Hijri) calendars side-by-side?
Windows 7 doesn't support this feature unfortunately. You'll probably have more luck looking for calendar gadgets. For what it's worth, Google Calendar has a wide variety of calendars you can subscribe to, albeit being a web app, rather than a desktop app.