Some fake antivirus like Antivirus 2009 or MS Antispyware 2009 will modify userinit.exe and after you have removed the fake antivirus with removal tools, the userinit.exe cannot be recovered.
We need to get it from Windows XP CD, but the Windows XP CD only provide the old version of userinit.exe only. The one I uploaded is the latest one. I do not use SP3, as there are many bugs will happen when upgraded from SP2 TO SP3. Thus, I still use Windows XP SP2.