Other solutions may work, but they may not be suitable for all situations. In CMD or PowerShell, you could stop listening to port 80 if IIS uses it (which it usually does)
iisreset /stop
Start over (after you've finished working with MAMP)
iisreset /start
A better approach would be to change Apache's port. To do this, you need to change the port in the httpd.conf file, which you can find in the [MAMP installation folder] under "conf\apache" and search for Listen 80 string.
MAMP app can be used to change port instead of previous solution. Under "MAMP > Preferences" menu, select the "Port" Tab and change the Apache port to, for example, 8080. It is also possible to change the port of Nginx and MySQL.
It is best to open "MAMP > Preferences" and then go to the "Port" Tab and click the button labelled "MAMP Default". It will set all ports to proper values.