FYI, Windows does have a chron "equivalent". It is called Task Scheduler. It has both a Windows and a command line interface. Something like "run program X every 30 minutes" is possible. I've used Task Scheduler to periodically run a Perl script before without problems.

Update with a few comments and considerations: I wouldn't necessarily interpret the user request for "one program" as a hard and fast design implementation limitation. It appears that the main point appears to be something reliable. Simple often, but not always yields reliable.

For the "wait for emergency" function, you could have a simple loop that does blocking I/O on that port while waiting for the "Emergency" message. If you get messages fairly regularly that are not "emergencies", blocking I/O would fine fine.

For the once per 30 minute "watchdog on the monitor for emergency" function. In applications that I've worked on, more than the "process is running" is often required. The thing to be verified is: "is the thing that is monitoring for an emergency" really doing actual work(processing messages looking for an emergency)? You could have the main monitoring function do something that is observable by the "watchdog" process. Perhaps simply incrementing the name of a zero length file for every message it looks at? If the watchdog function determines that no messages are being processed by the monitoring function, it would conclude that something is amiss in addition to making sure that the monitor process is actually "running", i.e. in the process table.