computer boots wrong hard drive

My wife's computer has started once in awhile to try and boot from the d:\ drive. We turn off the computers at night. Some mornings when I turn on her computer I get the _ flashing in the upper corner of the monitor. When I reboot and go into the setup. I go to the boot section and the d:\ hard drive is set as the primary drive. So I have to reset the c:\ hard drive back to the primary. Then everything is fine. I don't know what's causing this. She has the latest win 10 64-bit. And it started happening 2-4 weeks ago. And it doesn't happen all the time. I'd like to know why this is happening, and how to fix it. The motherboard is 7-10 years old.

Looks like the bios isn't retaining settings, are the date and time always correct when you enter the setup? If not you probably need a new CMOS battery, 10 years is pushing it without replacing it.