Do I need to buy a firewall? The website networksecure247.com is trying to sell me one. They call and say that "alerts" keep coming up on my network. I have a 2013 Dell computer, and the firewall from Microsoft expired on August 16. This company, Network Secure, is telling me that I need a firewall. Do I?
You needn't buy anything from someone who calls you unsolicited and tells you need to buy something from them.  In fact, you're generally better off not buying things from people who call you unsolicited.  In fact, you're generally better off not even listening to people who call you unsolicited to sell you things, and just hang up on them instead.  Particularly when they lie to you about things like "alerts keep coming up on your network" which is patently absurd.

The Microsoft firewall does not expire. Not on 8/16/16, or on any other date. It is a component of Windows and is good for all eternity, or as long as your computer lasts, whichever comes first.  There is no need to buy an additional host-based (runs on your computer, in other words) firewall.  
