You could try mon. It is a server/client package designed to do what you want. You still have to write some scripts to do the actual tasks, but mon will make it easy for you to run the monitoring scripts at defined intervals, and take the necessary actions depending on the monitoring script return value.

With mon you wouldn't have to worry about the scheduler/alert part, and focus on writing the actual "plug-ins" that will query the servers and the alert scripts that will page you or send you messages on your cell phone, using the advice given by the other monks...

hope this helps,

update: I haven't used mon to monitor windows servers, but I don't think there will be any problems, since it is pure perl and you write the monitoring/alert modules.