Did you install Advanced Personal Firewall and Brute Force Detection? If you don't access your linux box from outside the network, did you enable and forward SSH (port 22) or telnet on your router to your linux box? Have you re-routed your SSH port to a different port? Have you done a "yum update" since you last installed the linux? Are your root password alph-numeric and at least 12 characters long? Did you secure your "/tmp" directory? Are you running suexec?
Seams like a large task, but even if you have done the things above, there is nothing to stop a master hacker.