There is a good chance that the spyware you had on your computer did something to corrupt your Windows Defender installation. You can try and run the Windows System File Checker and see if it will fix the issue with Windows Defender on your computer. The Windows System File Checker will scan your computer for corrupt or missing system files and replace them as needed. You may need to have your Windows DVD handy in case it asks for it if it needs to copy any files.
If you got this spyware\virus on your computer not too long ago then a System Restore may be the way to go to get rid of the spyware and get Windows Defender working again. It will restore your system settings to the date that the last restore point was made. It won't change any of your documents or other files. Just be sure to choose a date for the restore that was before you got the spyware or virus.
As a final option you may want to try Microsoft Security Essentials as a replacement for Windows Defender. It's a free download from Microsoft and does basically the same thing plus more.