The concept of Debian stable is kind of nice, since you don’t have to be affraid of upgrade breakages all the time, but it can also be annoying, because things like spamfilters and virusscanners are fast-moving and you often end up with old versions of those. That is why the Debian volatile project exists.

On new installs, the packages sources will already be in /etc/apt/sources.list, but the sources.list doesn’t seem to be upgraded on old systems, so you may need to add them by hand. These are the sources: 

deb http://volatile.debian.org/debian-volatile lenny/volatile main
deb-src http://volatile.debian.org/debian-volatile lenny/volatile main

It’s speaks for itself that you have to change lenny to whatever the release will be…