Ports on the internet are like frequencies on your radio. They allow you to use the same "air waves" for a lot of different communication. So, basically, if you try to connect to a service on the internet, like a webserver, then a webserver needs to be eagerly waiting for your request. When you type in HTTP on a browser (along with the URL), your browser assumes you want to connect to the destination Webserver via port 80 (unless you override it). Since Campaign Enterprise has a webserver built into it, it listens for requests. If you put Campaign Enterprise on the same computer as an IIS Webserver there is a possibility that the two might collide if they are both trying to listen to port 80. Actually, what will happen is that the first service that starts up will "win" and take over the listening of port 80. The second service that starts up will try to listen to port 80 and fail because the first service has "binded" to that port. You can see the problem: If two services were allowed to listen and respond to a request, the results would be chaos.
Since port 80 is well-known for website traffic it is almost always let through firewalls, etc. and should be used for response functions. Using port 80 in links within emails also are much more acceptable to SPAM ranking filters than a hyperlink using a "strange" port like 81.
To make Campaign Enterprise use port 80 but "play nice" with other Webservers on your system, you will have to assign multiple IP addresses on your computer so that Campaign Enterprise will bind to one IP and the IIS Server can bind to one or more of the other IPs. It is the pair of IP and Port that makes the traffic coming back to the computer unique. If traffic comes in directed to the computer using the IP and port that Campaign Enterprise is "listening" on, then Campaign Enterprise will respond to it and the IIS Server will ignore it. The opposite applies to the IIS Server traffic where the IIS Server will respond to it's IP and port assigned to it. So overall, if you find Campaign Enterprise when starting says "port in use", you will probably need to make adjustments to Campaign Enterprise and the IIS Server to allow them to co-exist.
"So", you may ask, "why does Campaign Enterprise come with the default ports of 81 and 82?" This is done strictly for demo purposes so that you can be successful demoing the product on a computer with IIS Server running without running into the problems of colliding ports. When you are ready to use Campaign Enterprise for real, change the response port to port 80 so your email will not be marked as SPAM because of the strange ports used in the URLs.
There are two different ports used in Campaign Enterprise, one for the administration of the product and the other is for the traffic coming back to the server. It is probably good to keep the administration port something oddball if you are planning on allowing administration of Campaign Enterprise from outside your firewall because then it is less likely to be crawled by robots out there looking for computers to harass.