Comment by kotaKat

I know of a hospital still running a Meditech MAGIC system for historical data purposes. The system is a collection of IBM servers, all running Windows Server 2000.

Meditech, however, uses its own network stack on the OS, replacing all Windows networking components with a Meditech MAGIC driver, and Meditech's own software then controls TCP/IP inside its emulation that runs on top of 2000 in usermode. It's amazing how well put together the stack is that this could be (fairly easily) migrated up to a Server 2012-era box* and still work the exact same way.

*(that I know of - I assume they have newer support by now)