Just undo the FIRST of the Registry changesd listed in this article -- you change the OEMBackground value from 1 back to 0 which causes Windows to not look for any OEM Background picture and just use the windows default. There is no need to undo or delete anything else.

Yes, people who are comfortable in the registry AND compulsive about keeping the registry cleaned up and deleting unnecessary files might want to undo everything else done in this article, but there is no actual need to "fix" anything. The single registry value OEMBackground controls whether Windows uses its default logon screen or looks for a custom screen.