Now for the simple answer. If you're scanning from across the internet and port 80 is showing as open, don't worry about it. Lots of ISP's use transparent caching on port 80, and this is why port 80 appears to be open when in fact it's not.

To prove that port 80 isn't really open, try starting apache on the machine you scanned. If 80 is in use, apache will complain that it can't bind to the port and will refuse to run. If it starts and you can retrieve web pages from that PC, you can be sure it was appearing as open because of transparent caching.

To be doubly sure, you could use the perl script at www.cgi101.com that returns all your environment variables and view the page from the PC you used to scan from. If you don't understand what they mean, post the environment variables here, as transparent caching is very easy to spot from environment variables.