Som, can you tell us a little bit more about why you should check to see if it's open?
Port 80 is that particular port which is most vulnerable to:-
711 trojan (Seven Eleven), AckCmd, BlueFire, Cafeini, Duddie, Executor, God Message, Intruzzo , Latinus, Lithium, MscanWorm, NerTe, Nimda, Noob, Optix Lite, Optix Pro , Power, Ramen, Remote Shell , Reverse WWW Tunnel Backdoor , RingZero, RTB 666, Scalper, Screen Cutter , Seeker, Slapper, Web Server CT , WebDownloader.
Moreover during Windows Product Activation (WPA) uses the following ports:
80 for HTTP
443 for HTTPS
To test if port 443 is open:
1. Start Internet Explorer.
2. Type the following into the address bar and press Enter:
3. If the Microsoft page opens in the browser, port 443 is open. If the connection timed out, traffic through port 443 may be blocked.
NOTE: WPA will issue Error 32777 if it can't connect to the Product Activation server.
NOTE: WPA will issue Error 32777 if you computer's date or time is incorrect.
I hope this will help.