Even if on the official page for this laptop you can only find drivers for Windows 8, Windows 7 can be installed and can run fine.
To enter Bios and set the DVD as the primary boot device, spam the Delete key while keeping Shift pressed.
Drivers
Asus X555LD comes with a CD with drivers. However, you won’t be able to run the auto-install for the CD.
In order to install the drivers, press the Start button > Computer, right click > properties.
Device Manager. You will notice all the missing drivers.
Now, with the CD in the DVD-ROM, right click on any missing driver and click on update driver.
Select the CD where the software will look for the driver.
You will be able to install most of the drivers like this (manually).
For the wireless driver, you will have to browse the files from the CD. The wireless driver for Windows 7 is located in software > wlan > Atheros. Install that driver.
The VGA (integrated video card) and the GeForce driver can be downloaded from the official page. (Select Windows 8 and install them like that).
After you install the video drivers, you will be able to set the luminosity which is disabled without these drivers (the bar from where you adjust the luminosity is missing).
If you have any questions, please leave in the comments.