Ubuntu 9.10 Karmic Koala is out. There is a good chance that you are planning to run Ubuntu along with the existing windows installation. In such a case, it becomes hard to free up space, move data and create a new partition for installing Ubuntu. The solution lies in using Wubi.
Wubi is a free open source application licensed under GNU and is an acronym for Windows Ubuntu Installer. For information on Wubi, visit this post by Keith.
Using Wubi is pretty simple.
It prompts for a username password, installation location and size.
There are options to select the desktop environment and the install language as well.
After installation is complete, this install location is mounted as a boot disk and presented as boot option when the system boots, allowing you to dual boot into either your native Windows system or the Ubuntu system you just installed.
The biggest advantage is that with Wubi, we can install Ubuntu anywhere. There is no need to manually change the bootloader, edit the boot process or create any partitions to dual boot.
The only drawback is a slight reduction in disk performance which can be overlooked if you are not performing continuous disk I/O.
Download Wubi here.