Some threats require special treatment or procedures to remove them so the better software won't remove them by default but report them so the owner/use can do more research about the pest and its removal.
By changing the defaults you forced the removal and announced to all "I know what I am doing" and now see why more research was required.
Recovery of a busted XP OS varies with what XP CD I have and whether or not the owner has backup copies of what they can't lose.
My Windows XP machine was recently infected with I-Frame-Info. I read about malware removal software on Cnet and Malwarebytes is highly recommended. So I installed the free version and ran the quick scan. Results showed many threats but not all of the were checked for removal so i checked them all and selected Remove. My computer will not start now. It gets stuck on the Windows logo screen. So far I have let it remain in that state as I am not sure if it is okay to power off and try again.