Upon starting the machine I get an error message "Selected boot image did not Authenticate". I've tried all the forum fixes I can find to no avail. Many of them don't even yeild the same options for potential resolve.
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As I understand you are getting an error message and you are no longer able to boot into windows. Don't worry as I have a few steps which should help you to resolve this issue.
To provide you with an accurate solution, I'll need a few more details:
Have you made any changes on the computer before the issue started?
Was there any update/reset interrupted recently?
When was the last time it worked (date)?
In the meantime, let's try these steps here:
Perform a hard reset (Remove all cables from the AIO, hold the power button in for 30 seconds, reconnect the cables and restart the PC)
The information I can find about this error indicates that it is related to secure boot which is a new security feature launched with Windows 8. This error could have occurred after disabling Secure Boot. Try the following.
Reboot the notebook and press F10 until the BIOS loads.
Check both the "boot options" and "secure boot configuration" sections for the ability to disable Secure Boot and enable Legacy Boot Support.
Disable Secure Boot and enable Legacy Boot Support.
Save the changes and reboot the notebook.
Verify whether the issue is resolved and post the result.
If the issue persists, next step would be to run the Startup test. This will test all the hardware in the system and detect any problems.
If all tests pass you may want to RESET windows.
Here is a link to restore, refresh and reset windows, you may want to try the restore and refresh first, as they will not delete your personal files.
If you are still unable to load into windows. I would personally suggest this could be a hardware issue. I would recommend you contact our HP phone support for available service options. Please fill in the product details to get the tech support number.
Please use the following link to create yourself a case number, then call and it may help speed up the call process: