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
To force update the ClamAV virus database in the terminal you can use the command
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