You do not need anti virus or anti spyware on Linux. Linux is more secure then Windows, it is permission based system, which means so long as you are not in what is called Root Permission, you cannot mess up your system because you cannot write to certain parts of your drive. Now for an outside source they generally cannot even read your drive.
You can set permissions to different folders and parts of your system however, but Linux has a pretty darn good firewall and does come with virus protection mainly for emails and such(designed for windows, so you don't unintentionally infect a friends computer). But I use downloaded software on my Linux all the time, that when I use with my Windows it say it is a Virus and won't run. Believe me Linux is impervious to the 99.9%(I say 99.9% because anything is possible, but should say 100%) of all the viruses out there. The only way anything can effect your Linux system is if your Root password gets hacked(don't give it out, or make an easy one) no viruses can install or do a thing to your system because of permissions.
p.s. a lot of servers(90%), possibly even the ISP servers you are connecting too use Linux, now if they used Windows the Internet would probably crash daily. Love the Linux virus link from X, ain't that the truth.
Downloaded countless viruses that my Anti virus on Windows finds but doesn't even do a thing on my Linux system. Linux is immune, haha. Believe it or not.
Servers and Supercomputers section of this page: