I hope this microcode workaround works with the new microcode update. Here is the source link material. http://www.tenforums.com/installation-setup/11429-windows-10-will-not-install-system-pentium-g3258-cpu.html
1) Either disable one core of the CPU in the BIOS or remove all OC and return to stock settings or both. Whatever it takes to install Windows 10 successfully.
2) Once booted into Windows 10, navigate to C:\Windows\System32 and find the file named 'mcupdate_GenuineIntel.dll'.
3) All you have to do is rename it (most posts I've read say it's safe to just delete it but I prefer renaming it), such as adding .old or .bak to the end of the filename. So it would look like 'mcupdate_GenuineIntel.dll.OLD' or 'mcupdate_GenuineIntel.dll.BAK' (or 'mcupdate_GenuineIntel.dll.MyStupidCat' will also work )
4) You will have to temporarily change permissions on the file to change the name since it's a system file and will say 'File Access Denied' when you try to rename it (or do anything with it, really)
Change the permissions:
(Credit: © 2015 Sergey Tkachenko at WinAero.com)
There are pictures on that site if you prefer to go there and do it that way, but I'll just describe it with steps.
This may seem like a lot of steps but it's all the steps necessary for someone who has no previous experience working with file permissions. For those who are, just set permissions as usual to allow renaming of the specified file.
1) Right-click on the file whose permissions you wish to change ('mcupdate_GenuineIntel.dll', in this case)
2) Select Properties then click on the Security tab.
3) Click on the Advanced button. A window named 'Advanced Security Settings for Data' will open.
4) Here you need to change the Owner of the file. Initially it should say the Owner is 'TrustedInstaller'.
5) Click the blue letters just to the right of that that says 'Change'.
6) A small window named 'Select User or Group' will open.
7) You will need to select a User or Group. There are a couple of ways to do this and this step may cause some confusion but here's how I do it:
a) Type your name or whatever the name is of the current user (click the Start button at the very lower left of the Desktop and the name is at the very top of the left column) into the box that is named 'Enter the object name to select'. The spelling must be exact.
b) Click on the Check Names button. It should then show the name of your computer plus the name you just entered.
c) Click OK.
8) Now you will provide yourself full access to the file. (which will allow the renaming of the file without the Access Denied message)
9) Close all open dialog boxes by clicking OK on each of them.
10) Right-click the file (mcupdate_GenuineIntel.dll) once more. Select Properties then click on the Security tab.
11) Click the Add button and the box from before named 'Permission Entry for Data' will open.
12) Click on the blue letters that say 'Select a Principal'. Once again the small Select User or Group box will open.
13) Again type in your name or the name of the current user (same as before) and click Check Names then OK.
14) This will take you back to the box named 'Permission Entry for Data'.
15) Make sure there is a check mark in Full Control then click OK and OK again to close all open dialog boxes.
You can now rename the file.
After renaming it, reboot the computer and enter the BIOS to re-enable the CPU core that was disabled which allowed the installation of Windows 10.
You can now also re-apply any overclocking settings that you wish if you do so inside the BIOS. I prefer this method but it's not necessary at this time if you use a utility such as Intel XTU (Intel Extreme Overclocking Utility) to set overclock settings from within Windows.
Windows 10 will now boot normally with both cores of the G3258 CPU enabled and any overclocking you wish to add.