Hello Puzzeld,
Windows Defender is a anti-malware program that is a built-in feature of Vista and cannot be uninstalled. I would recommend to leave it running since it can help protect you and will not hurt anything to do so. However, if you still wanted to, you can
turn off Windows Defender.
This will help show you other methods of accessing and changing your startup programs in Vista though.
Startup Programs - Enable or Disable
Hope this helps,
Shawn