Try F8 at boot up and choose 'Last known working configuration'.
Does that sort it?
If not, stick your Windows disk in the machine and restart.
Choose 'Repair Computer', but as soon as the process starts, cancel it or you will have to wait a long time.
Do whatever it takes to close the 'Repair' process, you will probably have to agree to a restart that doesn't come but you will end up at a screen full of options.
You should note that you can get to command prompt this way too.
From the list, choose 'Restore', select a date that pre-dates the update and execute the restart.
If it starts now, then you should go and disable Automatic Updates. (Which is something I've been saying all along.
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Any help?