Purely speculative, Kat, but the error message saying that Uninstaller can't initiate sounds like the "Windows InstallShield," the normal "handler" for software installation and removal, thinks that Eudora is already on your machine and is trying to take it off.
If you copied Eudora files, from any version, onto the machine, or made an unsuccessful prior installation attempt, the installer may be recognizing Eudora files, and looking for the uninstall files, or key files it normally removes - which aren't there.
In most normal software installations, Windows makes "uninstall" backup files, for use if you want to remove the software later. Disk cleanup will ask if you want to delete them to save space, so they often go away.
It may be that new installations of Eudora attempt to remove old versions before installing themselves, especially if you're installing to a "default" folder containing prior version files. Some programs do this. If the installer thinks the program is already there, and can't remove it, the whole new installation may fail.
As I said, this is pretty much guesswork; but I've seen a few, fairly rare occurances. Usually an installation - where this happens - will proceed if you can move the right old files out of the folder(s) (maybe just rename the folder and let the new install create a new one with the name it wants?).