ClamTK is the graphical interface for ClamAV, you cannot use ClamTk without having ClamAV installed previously.
Once installed ClamAV will run in the background as a daemon and you can use the ClamTk interface in Linux to interact with it.
The ClamAV database is automatically updated via the running clamav-freshclam
daemon started by the /etc/init.d/clamav-freshclam
init script, there is no actual need to update it manually.
It is controlled by the /etc/clamav/freshclam.conf
with a default configuration of checking for a new database every 1 hour
# Check for new database 24 times a day
Checks 24
DatabaseMirror db.local.clamav.net
DatabaseMirror database.clamav.net
To force update the ClamAV virus database in the terminal you can use the command
sudo freshclam
ClamAV is definitely a good anti virus software, for a list and more information about anti virus for Ubuntu please have a look at any of the following posts