1. Open BIOS (upon restart press F2 for Acer, other model might use different key such as F6, F12 and DEL)
2. Look for setting which allow you to change from EUFI BIOS to LEGACY.
3. Press F10 and then hit ENTER.
4. Upon restart press F12 and select your bootable USB stick from the Menu.
Now you can proceed with Windows 7 installation.