The easiest way to get notified in real-time whenever a Windows-based system boots is by forwarding "Event Log" event 6009. This event is logged to the System event log whenever a Windows OS starts up.
Event Log: System
Event ID: 6009
If a real-time notification is not required then you can also schedule an uptime report in the web reports which includes all hosts that rebooted (including network devices) along with additional details. This report is available under Health -> UPTIME -> Recent Reboots.