Dear Jonas,
You may have to perform a custom install (click on the "Customize" button in the installer). There is no need to uninstall Xcode 2.5 first.
Thank you for yor suggestion. I tried that, and the installer is unaware of the fact that I already deleted the entire Xcode directory. It still thinks I have all the files installed, and it gives me no options for installing the files. I think I generally messed things up with a combination of deleting that Xcode directory and running cctools, and now I have to figure out how to get it sorted out again, short of re-installing my operating system.
Yours, Kevan