ClamXav is no longer freeware. The recently released ClamXav v2 now costs $29.95 for a personal non-commercial license.
If you have an older version of ClamXav installed, you will likely be prompted that an update is available. If this update notice comes from the app, a note is displayed stating you must pay for the update (which many people have overlooked)… but if this update notice comes from ClamXav Sentry, there is no indication that a purchase is required. After updating to ClamXav v2 you’ll be prompted to either activate a free trial or buy a license.