To monitor a system that doesn't have an agent installed, you need to piggyback another agent. Thankfully, this is easily done.
The following instructions presume that you have HQ running and at least one agent properly set up somewhere. You can even run an agent on the same system as the server if you like, which is what I do - I use the server's agent.
Log into HQ via the web page. At the top, click "Browse Resources". Select the agent/system that you want to perform the monitoring. This is NOT the system that you want to monitor port 80 on, but another agent somewhere else. Click on the "Inventory" tab. Under the "Services" section, click the "New" button.
From here, give a name to your monitor, like "Webserver 3", or whatever it is that tickles your fancy. From the service type, select "HTTP". Now, click "OK", and you should be dumped back into the inventory screen for this service.
Go down to the bottom of this screen, under the "Monitoring" section, and choose "Edit". You will need to fill in the following screen to taste, but at the very least you will need to set the target hostname "webserver.mydomain.tld" or whatever, and you might want to enter something under the regex section to make sure you're getting back the page you should be. For instance, if you have a string on your home page that says "this is the Humdinger site", then in the regex section you could put "this is the Humdinger site" so that HQ will check to make sure that's there. The reason for this is if the site answers, but with an error message, you want HQ to let you know that the site is not feeding the proper page(s).
Now, click "OK", and you're done. Create an alert definition if you wish at this point. There are plenty of neat little tweaks and things you can perform, but this should get you pointed in the right direction.
