As of today, all ClamAV 0.94 or older installs will stop working. The Debian project has not released updates for their current stable release (5.0) yet to solve this, but there is a workaround. The Debain volatile project provides newer clamav versions. The installation steps to install ClamAV 0.95.3 on Debian lenny are:
1) Add the debian volatile repository to the sources list:
echo "deb http://volatile.debian.org/debian-volatile lenny/volatile main contrib non-free" >> /etc/apt/sources.list
2) Update the sources and install the new clamav package:
apt-get update
apt-get install clamav
apt-get -u upgrade
3) If you use amavisd-new (e.g. if you use a ISPConfig 3 server) then restart amavisd:
/etc/init.d/amavis restart