I presume this is in Vista not XP there. One suggestion is the msconfig utility for disabling that and leaving it that way. The Defender is a quick MS goodie that doesn't really do much to start with. You see constant prompts for updates. But a freeware like Spyware Terminator will do for catching adwares/spywares.
Make sure to disable the Windows firewall if that is enabled since that will interfere often with installations. Go into the security section found in the ocntrol panel for that.
