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 [http://www.top-password.com/guide/reset-windows-password.html|reset a lost Windows password] with a boot media:
# Install ISO2Disc on a PC that you have access too, could be a friend's PC.
# Get a USB flash drive (1GB or more required).
# Download the ISO image of PCUnlocker, and then open up ISO2Disc to burn the ISO file to your USB device.
# Boot your Dell computer from USB, and you can then reset forgotten Windows password.
Or backup your important files and then reinstall Windows.