Should I use Windows firewall if I'm behind a router

Quick question: Is it necessary to use the Windows firewall even though I'm behind a router?

I would say yes. While the router will protect your network at the point of entry, you want some sort of control within your network. I think that most people would argue that the Windows firewall offers no protection at all but I would say that it offers minimal protection. I've tried other firewalls like comodo and zone alarm but have found them to be too resource intensive. I use the Windows firewall within my network.

Unless you have close all non used ports your router is designed to let things flow back and forth . a fire wall is another point you can do this on .

You could also check out linux based firewall use an old pc as one ( best option )