You need to add
deb http://ppa.launchpad.net/ubuntu-clamav/ppa/ubuntu hardy main to your

/etc/apt/services.list and run apt-get update. After that apt-get upgrade clamav

If apt-get upgrade does not work, do apt-get install clamav clamav-base clamav-daemon clamav-freshclam

Update Virus Definitions

Use freshclam.
You will see an output like this:

[email protected]:/etc/clamav # sudo freshclam
ClamAV update process started at Wed Apr 27 00:06:47 2005
main.cvd is up to date (version: 31, sigs: 33079, f-level: 4, builder: tkojm)
daily.cvd is up to date (version: 855, sigs: 714, f-level: 4, builder: ccordes)