If you are running from cd, you can install software using the knoppix icon and selecting install software. You can select intalling an antivirus.

If you are running from hd, you can install clamav

Code:
#apt-get install clamav
It can detect some viruses and can be usefull. You also can download "BitDefender antivirus" , it is free and there is a debian package you can download.