When I click "Try elementary OS without installing" or "install elementary OS" my computer boots to Windows 7.
I have burned the elementary OS ISO in my USB.
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Just check a few things.
Boot your USB and complete basic setup until you reach partition manager. Select the last option "Something else" to proceed with partition manager.
In partition manager,
IMPORTANT..! Select the Windows Boot manager partition (EFI) from the drop down at the end of partition of table. And then click INSTALL.
Click Next and complete the setup. You will recieve a dialog box as REBOOT once installation completes. Just reboot and unplug your USB media.
Enjoy Elementary Loki..!
IF IT STILL ENTERS INTO WINDOWS
Enter into BIOS and change the order of OS Boot manager to Ubuntu/Elementary as default. It should now load the Linux grub loader.
Try burning the usb image using Rufus https://rufus.akeo.ie/
It is a good usb burning tool and does all the important works for you. And check is your image of elementary is not corrupted. You can verify the hash of the file.
let me know whats your pc model
Also this might help you to understand the whole process
Try using universal usb installer for making a bootable usb: https://www.pendrivelinux.com/universal-usb-installer-easy-as-1-2-3/