Netboot.xyz is a bootable environment which provides a way to choose, download and run various installers or utilities - 16+ Linux distros, AVG Rescue CD, Clonezilla, GParted, DBan, Partition Wizard, more - from a pre-boot menu.
To get started, download the ISO we've linked to here, and burn it to a CD or DVD, create a bootable USB key, or assign it to your favourite virtualization tool.
Boot from netboot.xyz and - before Windows has booted - the build we've selected will automatically connect to http://netboot.xyz and display your various options: "Boot from local hdd", "Linux Installers", "BSD Installers", "FreeDOS installers", "Utilities", "iPXE shell", "Network card info".
If you want to install Ubuntu, say, you'd select Linux Installers > Ubuntu, choose your preferred release and install type (Install, Rescue Mode, Expert Install, Specify preseed url). Netboot.xyz then fetches the appropriate image from the project directory or a trusted mirror (important for security), your system boots from it, and you carry on just as if you'd booted from a local DVD or USB key.
If you're running a PC then you'll be most interested in the Tools menu, where you'll find applications to edit the Windows password, recover lost partitions, test RAM, detect viruses, run or restore image backups, securely erase your hard drive, and more.
Please note, we've linked to an ISO image with a dynamic bootloader, which is probably going to be the easiest to use for most people. But there are other options, including specific USB key builds, for booting from GRUB/EXTLINUX, or with static bootloaders which will prompt you for the necessary network info. If you need one of these, check out the main download page.