So you can defrag lets say D:\(hd2) but you cant C:\(hd1)... If so I will have to look into that.
Did you try to replace it in safemode? Try that if not.
Do you have system restore enabled? If so restore as far back as you can- the date befor you lost your scan engine.
I will try to gether more information for you. You could all ways back every thing up and use recovery and/or format:reinstall

(ouch I know)