UNetbootin allows for the installation of various Linux/BSD distributions to a partition or USB drive, so it's no different from a standard install, only it doesn't need a CD. It can create a dual-boot install, or replace the existing OS entirely.
* Microsoft Windows 2000-Vista, or Linux, or a USB Drive.
* A broadband internet connection to download the distribution packages (unless installing from pre-downloaded media, see below)
* Spare hard drive space to install the OS in (varies depending on distribution)