Peachesbackwards

Until the other day I've been using both Windows Firewall and my Wireless Router but my internet speed started to get slower which might be partially due to some work my Broadband Cable Company has been doing in the neighborhood.
The woman I spoke to in the wireless department their said if I already have a router for my wireless connection as part of their Netgear Modem/Router then that Router is good enough for a Firewall and by using Windows Firewall too everything is going through 2 firewalls and slowing down the speed. Sure enough, when I turned off Windows Firewall, my download speed via measurement from TestMy.net was incredibly faster. The bottom line is, am I sufficiently Firewalled with just the Router instead of the Router and Windows Firewall which slows down my speed considerably? Thanks for your help.