Wubi is a Windows front-end for lupin (http://launchpad.net/lupin) and ubiquity (http://launchpad.net/ubiquity). Wubi and lupin provide a user friendly installer which allows to install/uninstall Ubuntu as any other program, without modifying partitions, without replacing the bootloader and without having to burn a CD. Ubuntu installation is almost identical to a standard dual-boot installation both in terms of behaviour and performance. Wubi accomplishes the following tasks:

• gathers all information required by the installer, and provides an NSIS wizard to edit the settings (most settings are autodetected and genereally there is little to do other than choosing a new password)
• generates a preseed.cfg file to be passed to lupin
• downloads the ISO (supports download resume and multiple mirrors)
• sets up grldr (grub for windows)
• reboots the system so that ubiquity/lupin can take over and do the actual installation