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Several things to try. With this your computer will go back to the day it was purchased. No documents updates etc. Lets try this and if does not work, post and we will try method 2.
Many times during boot tap
Ctrl + F11 or F8 - Access Dell computers Dell System Restore (DSR) partition on many different models.
If one does not work try the other.
You may have a bad hard drive. We can test that too. If you do, you must replace it before anything works.