Windows defender is an anti spyware software that is automatically installed on numerous Windows versions but you are unable to remove it through your add/remove feature, so you will have to disable it instead.
Here is a simple guide on how to disable windows defender on Windows Vista:
- Open defender
- Start > All Programs > Windows Defender
- Click tools then options
- In the Administrator options uncheck the Use Windows Defender box
- Click Save (you may be promted for an admin password)
To restart it simply go through this process again and check the box.