The latest version of Ubuntu actually includes Wubi. It's now a supported method.
But I don't know if it actually is any easier to do it that way, and there are downsides to doing it that way. It's a breeze to set up a dual boot system. If you use the live cd installation, it's pretty much automatic. You don't even have to make partitions first. In the installation, you can choose to have it resize your Windows drive and tell it how much space to use for Ubuntu. Then after it is installed, you will have a boot menu called GRUB where you can choose to boot to Ubuntu or Windows.
If you are having trouble resizing your partition, your problem may be that you need to boot to Windows and defrag your hard drive.