Yes (from memory - I'm now on Win7):
Start | Run and type in services.msc. That will start Window's Services module which is a two panel window. On the right, scroll down the alphabetical list to find IIS and click on it.
On the left, you'll be offered the option to STOP this service. Click on STOP.
In the toolbar at the top, click on the Properties icon. Within the Properties pop-up window, there will be a checkbox to start the service when Windows loads. UNCHECK that box and close the Properties box. Close the Services window.
THEN REINSTALL Apache using port 80.
The reason for all this is that M$ installed IIS to start - using port 80 - when Windows starts. That effectively precludes any other http daemon from even being installed on your computer - unless you know to STOP IIS and prevent it's automatic starting.
Don't forget to UNinstall Apache so you can install it properly (on port 80).