There is a free open source Anti virus, called ClamAV. As you may be aware, TurnKey is based on Debian, so the instructions on the Debian wiki should do the trick for you.

Unfortunately, we don't have a packaged ClamAV Webmin module, but I note that there is a third party one available, which you should be able to install if you wish. Let me know if you have any problems.

I don't personally have much experience with ClamAV, but please feel free to post back if you have difficulties or further questions and I'll do my best to help out.

FWIW, it's pretty uncommon for a Linux server to get infected with malware (not impossible, but certainly unlikely - mostly because servers don't usually execute email attachments from random people or browse websites infected with malicious code). However, if you ever have concerns for the health of your server, then you can check out a couple of Linux anti-malware tools, rkhunter and chrootkit. Note that both of those tools may give false positives, so please check in first if you have any concerns (before you start wildly deleting things).

Thanks also for your kind words of appreciation. If you have any more feedback for us (particularly constructive criticism) please do not hesitate to share.