There are passwords that can be stored in the SYSTEM context that can’t be seen in the normal Credential Manager view.
Download PsExec.exe from
http://technet.microsoft.com/en-us/sysinternals/bb897553.aspx and copy it to C:\Windows\System32.
From a command prompt run: psexec -i -s -d cmd.exe
From the new DOS window run: rundll32 keymgr.dll,KRShowKeyMgr
Remove any items that appear in the list of Stored User Names and Passwords. Restart the computer.