Wubi is Windows based Ubuntu installer. With it, Ubuntu can be installed an uninstalled like any other Windows application.
Installing Ubuntu via Wubi does not require creating new partition, as the hard disk is a file on existing file system. This is not a virtual machine though, boot loader will mount the virtual disk image as a hard drive.
I have installed Ubuntu on my Acer Aspire One netbook and everything went smoothly. Once you open Wubi, enter required information and hit Install, the process onwards is pretty much automatic. Installer will also change Windows Boot Loader and add an entry for Ubuntu. Whole installation took about 30 minutes and required pretty much no user input at all. Once it was finished, computer rebooted and Ubuntu started up. I logged in and connected to my wireless network and everything was ready to go.
Despite the fact that its using virtual hard drive rather than partition, I havent noticed any drops in performance. This is a very safe and easy way to try Ubuntu. If you decide that its not for you, it can simply be uninstalled from Add / Remove Programs.