Then you need to check out Ben Armstrong's article.
Virtual Server provides a service where a user can be notified if a virtual machine is not responding. This is called the virtual machine heartbeat. There are two situations under which a virtual machine might not send its heartbeat. One is because the virtual machine has crashed - and no programs are running any longer. The other is because another program on the virtual machine may be using all of the CPU resources and not leaving enough CPU time for our code to be able to send a heartbeat message.
Read the full article,
here.