Avira is offering free detection and program updates and it’s done automatically within the program or as per schedule. You have an option to manually install definitions update too in case you have more than PCs to install definitions update. Avira is considered as nagware because it will display a pop-up window when updating the program.
If you are using Windows Vista, the User Account Control (UAC) is not triggered when opening, configuring or updating AntiVir. The only time the UAC in Vista is displayed is during the installation and when manually updating the definitions
(instead of using the built-in updater).
AntiVir 9 will use around 80MB (Commit Memory) on Vista which is higher if you will compare to its virtual memory usage on XP which is 58MB. When running a full system scan using the default scanner priority level, AntiVir will use more than 100MB of memory. It only took 38 minutes to finish a full scanning of the Vista system.