1) Click Start, All Programs and open Defender. On the Defender screen click Tools, then click Options. In the Options list uncheck the "Check for Updated Definitions Before Scanning" box. Click save.
To disable any windows notification, go to Control Panel, go to Security and then click Security Center. In the Windows Security Center screen click Change the way Security Center alerts me and select your desired preference.
2) To manually download definition files for Defender, see this page. BrightHub