Use the windows scandisk and defragmenter to prepare the windows drive. To be completely sure, unhide all the hidden files before defrag. (It is also good practise to tidy up all tose rogue temp files and so on as well.)
Leave all partitioning and resizing up to the Ubuntu installer. Make sure you have a backup of your files - the installers are pretty good, but not perfect.
If you want to spend money, partition magic is the usual choice. Use it for resizing only, creating new partitions is best left to the installer.
Note: your biggest gotcha will be if the windows is vista. This is a different kettle of piranhas.
Of course, if you can backup all your essential files like that - you don't need windows
just restore the files to ubuntu and enjoy the freedom.
