You wont find this message in a manuals anyway as it is from clamav and not ispconfig:
as you see this message contains also a link which explains it more in detail:
This message reminds you just that there might be updates available for your linux distribution. You can install them with the package manager of your linux distribution (which is not part of ispconfig nor related to ispconfig directly). For instructions on how to keep a linux system up to date e.g. take a look in the manuals of your linux distribution. For ubuntu you execute just these two commands as root user on the shell:
apt-get -u upgrade
And dont worry if the clamav error does not go away then as ubuntu 8.10 does not contain a newer clamav version, the antivirus signatures are still up to date an your system is still secure.