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Try starting in Safe Mode by tapping Function 8 before Windows starts to load. If that gets you in, go from the Start button to Run and into the Open box, type regedit then hit Enter.
From the File menu choose Export then note the name and location of the crucial backup of the Registry that you're about to create, just in case anything gioes wrong. It's actually worth backling teh Registry up once a month.
Then, navigate as you would through Windows a file system to HKEY_LOCAL_MACHINE>SOFTWARE>Microsoft>Windows NT>CurrentVersion>Winlogon and scroll through the list to SFCScan. The setting should be a 0 (zero) but your's is probably set to 1 (one) so make the change then leave the Registry from the File>Exit menu and restart the computer - twice.
That should stop it happening again.