Help with virus or spyware!

Recently my computer was infected with either spyware or a virus. I had a program that kept popping up called Windows AntiSpyware and it would keep saying my computer is infected and ask me to purchase the product. It was affecting my internet and my computer. I downloaded norton internet security and i deleted it, but now it seems it deleted some files i needed for my computer. I can not go on the internet without it asking what program i want to use to open files and I can not turn my firewall on. It says that firewall.exe has been deleted. Does anyone know what I am talking about and can anyone help me. It would be greatly appreciated. Thank you