I don't know if that last statement is completely true.
I use Norton SystemWorks, AdAware and SpyBot in Windows. AdAware and SpyBot have free versions and if you Google those names you can find the download site.
This is absolutley...correct. Macs at the moment are impervious to viruses (this may change in the future however, but I hope it doesnt!). When running Windows XP a Mac is a Windows-based PC-making it virus heaven. My family bought Norton (yes I know) so I just use that-its easier than me having to download loads of things. Also I kinda had to pay half cos I lost the old Norton disk (hehe...) so I might as well use what I paid for.