I've been running Linux on various boxes for years. For the most part, Iv'e either dedicated a machine completely to Linux, run Linux in a VM, or dual-booted. The dual-boot always required setting up a separate partition and messing with the boot.ini, or whatever, file after installing Linux into the new partition. Then I discovered the Wubi project.
Wubi lets you install Ubuntu into a virtual partition, represented by a file, and modifies your boot loader, adding a new entry. It will automatically detect your CPU type and download the appropriate version of Ubuntu (e.g. If you are running an AMD x64 chip, it will download the AMD x64 version of Ubuntu). You can select from four different desktop environments, KDE, GNome, XFCE, or the Mythbuntu media server.
After you install and reboot your machine, your new Ubuntu installation will appear in your list of boot options. All without the need to shrink and create new partitions.
If you are thinking about trying Unbuntu, but don't have the resources to run a VM or re-partition your machine, I'd encourage you to try Wubi.