Forgotten windows password for inspiron 3520.

Boot your computer in safe mode and check if you could login with the built-in administrator account. That account has no password by default.

Another way is to reset a lost Windows password with a boot media:

1. Install ISO2Disc on a PC that you have access too, could be a friend's PC.
2. Get a USB flash drive (1GB or more required).
3. Download the ISO image of PCUnlocker, and then open up ISO2Disc to burn the ISO file to your USB device.
4. Boot your Dell computer from USB, and you can then reset forgotten Windows password.

Or backup your important files and then reinstall Windows.

If you have a created password reset disk, you can use the disk to reset Windows password. If you don't have one, you may need a third-party program such as Windows password recovery to help you.

Such Windows password recovery tool can help you created a password reset disk, then you can use the disk to reset the forgotten Windows password with ease.