:S.. if the firewall windows comes with is slowing down your computer.. i doubt another firewall will be any better.
If anything the windows firewall should cause the least pull on your system resources.
If your on a router you don't need a firewall.. if your direct on a modem you still probably don't need one..
Avast is a free antivirus which comes with some inbuilt firewalling.. should be all you need
http://www.avast.com/eng/avast_4_home.html