The reason antivirus programs cannot find any virus is because the virus as such has been removed. What remains is a registry entry that it had created and hasnt been properly removed.
Go to
START -> RUN -> type CMD -> now paste the following:

reg add “HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon” /v Shell /t REG_SZ /d Explorer.exe\

the error should disappear on the next restart.