You'll need to do some registry tweaking and a have copy of INSTSRV.EXE and SRVANY.EXE from the Resource kit, do some googling you'll get the exact procedure. Theres a good number of 3rd party programs like the one Richard linked too around as well that will make it easier to setup.
I used FireDaemon a few years ago for an FTP server application I had that didn't have it's own service running on a Windows box. It was easy to configure and worked like a champ.
I have just updated the script so you can specify just to recieve a email when errors occur. Examples of error's that could occur that the program will notice are..
404 Error Page 500 Internal Error (ASP Script Error) Or just if your site does not respond within 10 seconds
Can anyone think of any additional options to add to this program to help improve it?