Yes, you don't need a virus scanner. Mac OS X is immune to all viruses, and you'll be hard pressed to find one in the obscure corners of the internet that work with Mac, let alone 10.5.5.
no need to worry about viruses as there are currently none for OS X. there are however a couple of trojans around the internet disguising themselves as QuickTime video codecs needed to view their content (usually porn). however they need your admin password to install so if you be careful with which programs you install and be a smart user there is no need for a virus/malware scanner.
if you do want one theres Clamxav which is free and opensource and seems to get good reviews. thats the only one i would trust.
My MacBook is loaded with Windows viruses. They don't affect my Mac. However, when my friend using his Windows PC connects to my Mac he always gets a virus!