Linux is more less used than windows...this is why ppl don't write viruses and trojans for linux but if more people start to using linux then i think that u'll see a lot of viruses for linux too .I have had a linux system for almost two years now and I havent come across any viruses yet. Nobody has spend $400 bucks for a bare bone computer and purchase an anti-virus program plus a subscription to protect it. It's unreal...
I think linux flavours are the future in terms of personal computing and the protection of information.
@everyone who says that his antivirus slow the pc:
If a security program has the side effect of slowing down the system then i think it's not so good , because it should have only benefical effect....instead this way maybe benefical effects are less than the bad ones.