what's involved in the Win 10 boot loader repair ?

I have a Windows XP/ Win 10 dual boot system, and I've also just moved to a replacement motherboard.

The Win 10 handled the different motherboard in it's stride (except for activaton issue, but that's another story)

The Windows XP *seems* like it is OK, but I'm getting some occasional crashes in the avast anti-virus and Firefox,
reinstalling them has not corrected the problem.

Because of this I was considering an XP repair install. If I do this it will more than likely restore it's own boot
loader, so I will not be able boot to Windows 10. I have copy of the bootable build 1607, but I have not found if it
is possible to find a recovery environment from the DVD

I know that pressing F8 in the normal startup can get you there; it accesses c:\recovery and I it runs from there.
Not sure if this is any use to me when the Windows 10 loader has been over written.

I feel like I'm missing something obvious. How is it supposed to work?

If it were me, I would install Macrium Reflect Free. Make a Macrium Rescue DVD, CD or USB flash drive. Make sure you can boot the computer into the Macrium Reflect Free rescue environment. Then back up the EFI System or System Reserved partition(s) onto a USB flash drive or other external storage location/device..

Do the Windows XP repair installation. Reboot the computer into the Macrium Reflect Rescue and restore the EFI system or System Reserved partition(s). If that doesn't work, Macrium Reflect rescue also contains a utility under the restore menu to fix Windows startup problems - that should fix things too.

Honestly, my response is just a suggestion, and I apologize if you think it's completely off topic.

Try to only use Windows 10 as your operating system. I'm sure every single program from XP now works with Windows 10. Windows 10 is way more secure than XP, because XP no longer has security updates.

Check this out though: If you want to create some sort of Temporary Workspace for you to boot into to do small things in there, and not have your Windows 10 reflect upon that, then look on the internet on how to install XP, for example, on a big enough removable USB stick, 8 GB or 16 GB or whatever. So whenever you want to use your "Temporary Workspace", plug in your XP USB and boot from it.

Thanks for the suggestions. It looks like I'm going to actually buy Windows 10, since after the motherboard replacement
it has lost its "digital license". At least I can step up to the 64 bit version now and install clean.

I picked up an Asrock 960gs-gc with n AMD FX-4300 installed. It was relatively easy to do a mini repair to get XP to boot.
There is a program on the Bart PE CD that sets the hard drive controller back to Standard IDE.
Once that is done the PC boots and I can then take care of the other drivers.

It was crashing in Firefox (xp), and after an automatic dump submission, they tell me in the forum it's something do with
AVX instructions. and an illegal operation. I'm wondering if the motherboard/CPU is not fully compatible with XP.
With that in mind, really don't know if an XP repair would even help.

Sorry to go off topic a little but I've been pulling my hair out on this. I guess i'm up against the clock with using Win 10
in it's non-activated state, so I have to make a decision soon. At least if I',m going to scrap the current Win 10
installation, I probably don't have to worry too much about the XP repair and the boot loader question.

An update. Problems with various program crashing in XP were solved by a repair install.

The issue of restoring Windows 10 boot loader is solved by creating the recovery USB drive. This can be done by
searching for the tool in the control panel.