Do you have FCP X installed already?
If so FCP 7 will not install.
Did you have FCP 7 on your computer previously and are reinstalling for some reason?
You did not remove all traces of it. It is much more than the software in your Applications folder -there are many files at system level.
Go here: http://www.digitalrebellion.com/fcsremover/ and use the tool to completely get rid of all traces of previous installations.
FCP X can be downloaded again at no extra cost but you must install FCP 7 first if you want both.