That's because the installer has detected you had already installed a copy of EB. In other words: you have not uninstalled at all, or you have not uninstalled through the control panel/ the installer. (EB is still registered on your system as installed, you see.)

Solution: choose 'remove'. Have a lot of patience nearby: the installer is going to remove all files it previously installed on your PC.

Then you can run the installer again (doesn't matter which one as the fault lays with the user, not the installer package...) and this time install to the right directory.

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There are a few other options to explore, if you wish, as well; but neither one of them will be as 'fool-proof', and 'user-friendly'. These options are - providing you have a spare back up of RTW 1.5 (which you can always create yourself)-
1) You could dump the EB folder created by the installer on top of it and readjust the shortcut to point at the right location. Downside: it will be as good as impossible to uninstall EB at a later stage using the uninstaller. And it isn't as clean regarding your registry files either (since something is registered there which doesn't actually exist anymore at the location it was supposed to be).
2) You could use 7-Zip or similarly powerful archive tool to extract the files to the right location. Readjust the shortcut as mentioned earlier. And remove the useless install of EB using the uninstaller too! Downside: this way the EB installer may 'think' there is no copy of EB installed on your PC at all.

It is probably a sound idea to defragment your harddrive after taking either approach.