As far as I understand you did install avast! mobile security and avast antitheft; after some time you have removed only avast! mobile security and now you are angry because you are getting update notifications for antitheft, which is still installed. So, as you may be have read, after installation of avast antitheft and turning it on, the application will hide itself so you can not reach it from app drawer. You can update the antitheft and use built-in uninstaller or you can install avast! mobile security and use uninstaller from there (located in Antitheft section). You should know your PIN which you have set during the first installation. Mobile security and antitheft are two different applications.