Today it is possible to install Ubuntu Linux 8.04 (Hardy Heron) under Windows XP or Vista without necessity to create separate partition for it. The tool that allows to do it is called Wubi. Actually it is Ubuntu installer for Windows helping you ro install and uninstall Ubuntu with few clicks. Here is nice brief article on how to set it up under Vista:
Wubi adds an entry to the Windows boot menu which allows you to run Linux. Ubuntu is installed within a file in the Windows file system (c:\wubi\disks\system.virtual.disk), as opposed to being installed within its own partition. This file is seen by Linux as a real hard disk.