2 big things: 1) watch 3rd party addons. research the vendor and look at comments on them at joomla.org 2) use your .htaccess go to: http://developer.joomla.org/security/articles-tutorials/260-joomla-administrators-security-checklist.html they have a lot of good suggestions for protecting your site.
Backup regularly, no matter what steps yo take, the best thing you can do is have the site ready to go back up if something does happen.
I would add to the fine suggestions already listed: Keep Joomla Updated! Everytime they come out with a security patch, make sure you impliment it as soon as you can. I've had a couple test sites hacked, I put up a default install with a few themes to check them out, or show to clients, and forget about em sometimes. I've come back to a couple that were a year or so old, and they had been hacked... no loss to me since they were test sites, but the only thing I could point to was that they were about 3-4 updates behind, and the hackers have certain footprints they look to exploit...
I will echo what the last poster stated. Keep your version updated. It is the best way to keep the bad guys out of your site. One other thing is to use a good ( safer )version of PHP and MySql. That is up to your webhost, but they should be happy to serve you as a customer who may want an update to safer versions of PHP or MySql.
u can optimize the antivirus tools on your cpanel provided by the webhosting where u placed ur website
For the OP, If your webhost has a virus...... Simply get another one, there are thousands to choose from. `