Task Scheduler is corrupt

I upgraded to Windows 10 (on a PC), but I was unhappy with it and downgraded again to Windows 7. After a few days I got the error 'GWXUX has stopped working'. I believe GWXUX is to do with the Win 10 upgrade. Thinking I could fix it, I tried to launch Task Scheduler. I now get a succession of error messages along the lines of: 'Task {FD779DF0-4047-4AFF-95E5-E267E2669CB0}: The task image is...