Most applications of this type are built to resist attempts to remove them from the hard drive, without the password. That's because one thing this type of application is
supposed to do is to prevent kids from messing with their computers.
One
general way to repair this problem is to
reinstall the application and give it a new password during the installation. Then, you can use that password to uninstall that. I haven't tried this with Web Defender, so I don't know if it will work or not, but it's probably worth a try.
Here's a link to download Web Defender (I hope I'm correct that Web Defender = Windows Defender).
Windows Defender: Home Page
"Plan B" is to reinstall the operating system or disk image and start over.