Problem

If you install Clamd and clamav on RHEL 5.3 (and maybe olderversions too ) or Centos the clamav rpms don't contain the unrar library so when you star clamd you will get this warning:

Starting Clam AntiVirus Daemon: LibClamAV Warning: Cannot dlopen: file not found - unrar support unavailable

Solution

The solution is really simple. All you have to do is install clamav-devel

and then restart clamd and everything should be ok.
Solution found thanks to this blog post ( yeah it's in Japanese and no I can't read Japanese but It was easy to figure out )