When a user types in your IP address or Host Name in their browser, the browser will look at the IP address on port 80 by default for the web pages.
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It is my understanding that many web browsers operate on a local port 80.
This means that not only do they connect to the web server on port 80, but open port 80 on your own machine. This might explain why your port 80 is open.
When someone attempts to connect to your FTP Server, the FTP client will attempt the connection on Port 21 by default. Port 21 must be open for standard FTP clients to log in and connect to your server.
If you run a Telnet server, this port is required for the Telnet Client to connect to your server. Telnet can be used for other ports to check services, but in order to use telnet for remote admin and login, port 23 must be open.
How to configure Zonealarm click here
File Sharing is enabled by default in XP Home.
I run XP with ZA as a firewall and have a wireless secured set-up.
As long as you have an antivirus program and antispyware then you are well protected