Why is my computer not booting to the correct operating system?

Check the BIOS settings. Look for an indication of the correct OS and make sure it is first in the boot order.
Check the hard drive partition. Use disk management to make sure that the system is set to boot to the correct OS partition.
Make sure the hard drive is connected properly. On some computers, this means double checking the connection to the main power source. On laptops, this means making sure the connection is properly seated in the hard drive tray.
Check the power supply. Make sure the computer is properly connected to the power source and that it is supplying the proper voltage. Also, check for any blown fuses or visible damage.
Verify the OS is installed on the computer. Make sure that the operating system is installed and not corrupted.
Check the RAM. Ensure the computer has enough RAM and that it is working properly. Also, make sure the RAM is compatible with the computer model.