Your system is infected with Worm.SdBot.eb virus.
Scan your system with latest virus definitioned antivirs in safe mode.
Start>Run>Regedit>ok
now navigate to:-
HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\RunServices
"System DLL Library" = "EXECDLL.exe"
  HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Run
"System DLL Library" = "EXECDLL.exe"
Delete the "EXECDLL.exe" keys.
regards,
S.Sengupta[MS-MVP]