The subject condition could be the result of an invalid registry entry.

NOTE: If you can't logon as an Administrative user, see tip 182.

Use Regedt32 to navigate to:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\Environment

If the right hand pane contains a Value Name of <No Name>, delete the Value Name.

Delete any obviously invalid entry, such as a type REG_DWORD value with string data.

Shutdown and restart Windows NT.