Changing ports

If you want to use for http other port than 80 you will have to use proxy server.You can find some free web proxy in internet or install your own in your local network.Then you will need to configure your browser to use proxy server, in IE you can make it by Tool > Internet Options > Connections > Settings and then write proxy server address and port ( for example my_own_proxy_server_address 8080)

Go to http://www.iana.org/assignments/port-numbers
This will tell you the well-known ports, which you will want to avoid changing the port to.  One thing you can do is try telnetting to the port you want to use, with a command like "telnet server_name port#"  If it comes up with a blank screen the port is active and in use.  If it says connect failed it is not in use and it's probably safe to assign to your web site.

instead of changing the port can I just assign the virus software to use a open one?

Does it matter which one or can I pick any number?

you can change the port you are useing without a proxy server. its the option for what port you use is in IIS although i dont remember the exact location off of the top of my head. i have done this before. when accessing your site if you use a port other than 80 you will have to specify the port to use in the url

i beleave i misread this post. i thought you were trying to change the port number for your http.