What dollie says is very wise, I rely on no single piece of software. I have quite a few software on this computer although not all is installed I do keep up to date setup files in case I need to use them. Software I have installed or have laying around ready to install for many applications is as follows
adaware, spybot, ewido, antivir, avg, trend micro anti spyware, rootkit revealer, hijackthis, about buster, cws shredder, killbox, unlocker, solo antivirus (good dos scanner), winsock fix(program I picked up thanks to bcastners faq on winsock) . Anyways as you see I use several , not all as I said are installed but I keep them around just in case. Also I use housecalls online scanner every so often to check behind things.