userinit.exe; rundll.exe [initialisation errors]

Without advance warning, at startup, after logging in to my ID, a blank screen comes up and an "application error" message saying userinit.exe failed to initialise properly. I can access applications via the Task Manager.

Some minutes after all this comes another message that rundll.exe failed to initialise properly.

I have investigated and found that the file C:\Windows\System32\Userinit.exe is present.

My internet connection is presently restricted and I would prefer to try to fix this without recourse to anything requiring a large download. Any help greatly appreciated.

My computer usage is pretty routine but my teenaged children also have access. Has one of them possibly imported something to cause this problem?

Bill.