In Vista - Windows Defender could not be found in Add Remove Programs so was unable to uninstall like many solutions to this problem stated. So I went to My Computer - Program File & found the file called MSASCui which is the Defender file. But when I clicked on it it would give the same error. I tried all the solutions found in the forums, & Microsoft, to no avail. Even trying to register the dll files showed some were not found.
Then my BF resolved it & I wanted to share the answer for anyone who is having this frustrating problem.
Right click on MY COMPUTER & click on MANAGE - in left hand pane click SERVICES & APPLICATIONS - click on SERVICES - scroll down until you locate WINDOWS DEFENDER - check the STATUS column and verify that the service is shown as STARTED in that column. Mine was blank so I had to double click Windows Defender and in the box where it says START UP TYPE: choose AUTOMATIC & click APPLY. Then click on START below that box & OK.
It solved the problem