Which Windows services are best for monitoring server availability

We are trying to determine which services are the best ones to monitor on Windows servers, especially Terminal Server and Exchange 2007.  

On our Terminal Server, we have been monitoring the NetLogon service, but that is not working for the occassional situation where people can logon to the server, but then the server "hangs" when loading their profile.  Is there a service that is better for monitoring whether the server is fully functional or not?

Thanks.  Greg.