If you need to uninstall MacOS X, and you don't want to/can't erase the partition, here is one of the better discussions on how to do it on MacFixIt:
Uninstalling OS X - Is it possible?
The key is making sure that the invisible files are deleted, otherwise on a reinstall of MacOS X you will be faced with a login window instead of the setup assistant.
Tip: Use Sherlock to delete invisible files: custom -> edit -> More search options, select 'is invisible', then drag the invisible files/folders to the trash.
Editor's note: You can also try this article on about.com's Mac OS X site for some additional tips.
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