The welcome screen is driven by msgina.dll, not lsass.exe. It's also possible that the registry entry that specifies which gina interface the machine is using is set incorrectly. You should be able to determine what gina the system is using from an offline registry editor (boot CD), and googling for the correct reg entry to inspect.
Also consider uninstalling SP3 using method 4 here:
http://support.microsoft.com/kb/950249
In the hopes that it will put things back to a sane state....