When installing Redgate Monitor you may find that it shows the web port is not available and there are several reasons this can happen. You will only see this when using the built in web server option, otherwise you'll be setting the port binding for the site in IIS.
First install
If this is the first time you are installing Redgate Monitor then another application is using the port. You may be able to find more information about what that is by running the following from an elevated command prompt (substitute in your port number):
netstat -ano | findstr :8080
This should hopefully give you the PID of the process using the port and you can find the name by running the following (substituting <PID> for the actual number returned e.g. 15824):
tasklist /svc /FI "PID eq <PID>"
You can then decide to stop this application or use a different port.
Subsequent install
For updating versions of Redgate Monitor after 10.1.0 this should no longer apply as the HKLM registry is now used to store the data used to pre-populate the installer (previously it was HKCU for the user that ran the installer).
If Redgate Monitor is currently installed and you are entering the same port as it currently uses, then it's likely now being installed by a different user than it originally was. The user registry is used to store the port and some other information when Redgate Monitor is installed so if it's being installed by a different user, the installer doesn't know that it's Redgate Monitor that's using the port as it doesn't have access to the other users registry.
In this case, all you need to do is to stop the currently running "Redgate Monitor Web Service" service and continue the installation.
Manually changed web server port
If you have followed this article to change the port the webservice uses and have not updated the registry entry accordingly then the webserver port will show as in use during the install, but it will be Redgate Monitor. You can confirm this by stopping the webservice or site in IIS. Whether you stop it or not, proceeding with the install the service will be uninstalled and reinstalled so the port will be accessible.