A 24-hr restart plan, sounds like a properly documented quality management process for a large boiler? Disconnecting the "Windows XP box" is removing the control system correct? Not really the same thing is it? There's no reason the XP Embedded hardware platform could not have been engineered with high availability in mind? It's not as if there are no comercial-off-the-shelf options here.

The company who chooses to let the bosses son, who built custom PCs in high school, design the critical process control systems, is I'd say, outside the scope of the argument here.