Sorry, I realize I should give more detail:
Windows 7, on a MacBook Pro running in Bootcamp. I currently have both Rhino 4 and Rhino 5 installed (For Rhino 5 I also have a 64 bit version), and I just double clicked the .exe file to start the installation from my downloads folder. Grasshopper is currently only installed in Rhino 4.
That's what I thought, but I downloaded it 4 times, and each time the download seemed fine, until I tried to install. Might it have something to do with my existing installation? To wipe out the existing installation, should I just delete the Grasshopper folder in plugins?
I restored the folder after deleting it, and tried to use the control panel uninstall, which returned an uninstall error, which makes me assume it was a problem with the uninstaller.
So then I deleted the folder along with every grasshopper.dll and GH_IO.dll that Visual Studio 2008 Express had copied into my VB.NET project folders. I tried to wipe every remnant of grasshopper off and try again with a fresh install, but I still got the same error.
Which means that there is either still an error with the uninstaller after attempting to wipe grasshopper off of the computer, or there is a problem with both my uninstaller and the installer, or simply that, as you said, there is some cached corrupt version.
If I could be absolutely sure that I had cleaned out the previous version, I would know if it is the uninstaller or the installer.
Here's the control panel error:
And I noticed that the installer always raises the error message when it is copying "Grasshopper.pdb"
I was thinking the same thing, and just tried the sep. 3rd version which worked, but raised a warning message saying that it might not have installed correctly, and when I clicked on the option to reinstall with settings for this version of windows, it (the sep. 3rd version) installed smoothly without errors.
And after reinstalling the 9/3 version, a renewed attempt to reinstall the version posted at the top of this forum still hits the same error.
And control panel uninstall of the 9/3 version raises the same uninstall error.
I might just reinstall Rhino.