Try this solution from http://social.technet.microsoft.com/Forums/en/w7itprosecurity/thread/e1ef04fa-6aea-47fe-9392-45929239bd68
Microsoft Support found the problem for us. Our domain accounts were locking when a Windows 7 computer was started. The Windows 7 computer had a hidden old password from that domain account. 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 toC:\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.