If you have things on the Ubuntu installation that you want to access in the future, then I would avoid Rory Alsop's suggestion; otherwise, his is the easiest way if you don't mind losing everything you have.
One nice thing about Linux is that the configurations, preferences, etc, are all in files that can be copied, unlike windows with its registry. So, if you are at all technically inclined (not necessarily a guru), you could use a live Linux CD, such as the Ubuntu CD, run gparted, and create a partition big enough for any data you may want to keep. If you have any Windows data you want to save, this would be a good time to do it while using the live CD.
Then, run the WUBI install, and copy the data, or your home directory, to the new partition, making sure to get the hidden files.
Once you do that, you can reformat the Windows Partition and install Ubuntu there (all from the installation program). Once you get everything the way you want, you can run gparted to delete the smaller partition, and grow the main partition to full size.