Depends.....
Click Start, Run and enter Msinfo32. Software Environment, Startup
Programs. The Command column will show you the exact command line used. If
you see one that uses a long file name/path with spaces in it, and its not
enclosed in quotes, you've found the culprit.
Go to Start/Run/Regedit and navigate to this key:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon
Change the key that reads:
C:\Windows\System32\Userinit.exe,C:\Windows\System32\Userinit.exe
To:
C:\WINDOWS\system32\userinit.exe,
This problem can be caused by the DlDer Trojan. More information here:
http://www.sophos.com/virusinfo/analyses/trojdownloada.html