If you are going to use clamAV with Kmail make sure that you do one of two things
1) Run freshclam at least once a day (preferably before you check you email if you are paranoid about such things as email virus for Linux)
2) Check to see if your *buntu ships with a freshclam deamon, if so then run that and it will automatically run freshclam every day for you. If not then I would add it to your cron.