If yours is a dual boot Windows 8 and Windows 7 system, you might have noticed that your computer restarts twice when you switch to Windows 7 from the new advanced boot menu. Of course, there are obvious reasons for that (you can read about it here), but you can skip this two reboot by making your Windows 7 as default operating system.
Step 1: Click Win + C keys on keyboard and select “Settings” from the Charms bar. Then click “Change PC Settings”.
Step 2: Now go to “General” and under “Advanced Start-up” click “Restart now” option.
Now, you will see a couple of options. Select “Use Another Operating System” from the options. It’ll display the boot menu. From the boot menu, just select Windows 7.
Watch below short video to know how it is.
Note: If you reboot your Windows 8 into Windows 7 in the normal way, it requires twice reboot. The above method will solves that trouble. [Via]