System Restore
1. Restart your computer. Before your computer reaches the Windows Vista startup screen, press 'F8' to enter Windows Safe Mode.
2. Click 'Start,' 'All Programs,' 'Accessories,' 'System Tools,' and then 'System Restore.'

3. Select 'Choose a different restore point,' then click 'Next.' Select from the list of available restore points the last date and time Windows Defender worked.
4. Click 'Next.' Review the information on the 'Confirm your restore point' screen to verify that it's correct, then click 'Finish.'
5. Allow Windows Vista to restore your computer to the previous restore point. Once System Restore is complete, open Windows Defender to check whether or not the program is now working.
System File Checker
6. Click 'Start,' 'All Programs,' then 'Accessories.' Click 'Run.' Type '%systemroot%' into the dialogue box. Click 'OK.'
7. Click the 'System32' folder, then search through the list of files and folders until you find 'cmd.exe.' Right-click 'cmd.exe.' Select the 'Run as an administrator' option.
8. Click 'Continue' if the User Account Control dialogue box appears. Type 'sfc /scannow' into Command Prompt, then hit 'Enter' to run System File Checker. If SFC locates any errors within Windows Defender, it will attempt to repair them.