This feature is called ClamXav Sentry, and this is what really makes this application valuable, it is a real time scanner that monitors changes in files and directories. It does require some setup and configuration to have functioning properly. Ideally you want to include directories where files are downloaded into your system for ClamXav Sentry to monitor. Alternatively, you can make it monitor your whole home folder if you don't know what you're doing.

Does not slow down computer under normal use.

You can roll your own engine if you wish.

Customer support does not cost anything other than a single purchase of the app itself.

Frequent updates means the user is more protected from current attacks.

Clam Xav has a one time cost of $29.95. There are no other fees associated with the app, since it is purchased, it is yours to use for as long a s you like.