In the configuration of AntiVir 9, you can find the following settings under General - Update:
Download and install product updates automatically (recommended)

Please note, that a product update may require an automatic system restart. This setting is recommended by Avira, because it ensures optimum protection against viruses. Certain product updates need to restart AntiVir services or the entire system. The complete security is obtained only after restart.

Without the automatic restart, the product's functionality, such as on-access scanner (AV Guard), is not available until an eventual manual restart.

In case you do not want to use this update option, you can change it in the configuration. In this case, deactivate the following options:

• Notify user if product updates are available
  Notify once again after X day(s)
• Do not download product updates

In the new Avira AntiVir Professional version 10 you can also find a new option under Configuration - Update - Product update:
Download product updates. If a restart is necessary, install the update after the system start, otherwise install it immediately.

This new option downloads product updates on your system, but if a restart is necessary, it installs the updates afterwards. In this way, AntiVir remains fully functional and your system is protected.
Please note, that a normal update (VDF or Engine update) does not require a restart, because it does not change any program components.