Thank you for contacting Samsung. For the most part, tablets are not vulnerable to viruses, however you can search the Play Store for antivirus apps. We can't recommend any specific ones, but there are a few out there.
As for Knox, if it is a pre-installed one, it cannot be uninstalled. If you'd like to check, you can do that by going to Settings > Manage Applications. Find it listed there, select it, and if the option to uninstall is available, you'll see there in the same area where you have the option to clear the data and cache.
If you have any further questions, please contact us at firstname.lastname@example.org and let us know how we can assist you further. Thank you for being a Samsung customer!
The Samsung Online Support Team