When I tried to listen on port 80 in Windows 7 with netcat, I get this error:
C:\Users\sashoalm>echo "hello, world" | netcat.exe -l -p 80 Can't grab 0.0.0.0:80 with bind
On the other hand, selecting any other port works.
It turned out this port was being used. I used
netstat -a -b to find out which program uses it (it requires elevated privileges).
It turned out Skype will use port 80 and 443 for incoming connections, after reading https://support.skype.com/en/faq/FA148/which-ports-need-to-be-open-to-use-skype-for-windows-desktop, I changed that and now it works fine, netcat can listen on this port.