Check the Userinit and PersistBrowsers registry entries to make sure that they contain the correct values. To check the Userinit and PersistBrowsers registry entries, follow these steps:
a. Click Start, click Run, type regedit in the Open box, and then click OK.
b. Locate and then click the following registry subkey:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon
c. In the right pane, right-click Userinit, and then click Modify.
d. In the Edit String dialog box, type Windows installation drive letter:\WINDOWS\system32\userinit.exe under Value data, and then click OK.
e. Locate and then click the following registry subkey:
HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\Advanced
f. In the right pane, right-click PersistBrowsers, and then click Modify.
g. In the Edit DWORD Value dialog box, type 0 under Value data, and then click OK.
h. Quit Registry Editor, and then restart the computer.
more at *support.microsoft.com/kb/899865