Moving this discussion to the Premiere Elements forum.
It appears that I resolved this issue myself, so I'll share what I discovered in hopes of helping others in a similar situation.
The problem occured after I had to reinstall Windows 7 64-bit, then all my programs. My copy of Premiere Elements 10 was purchased from a reseller, Amazon.com, as a digital download. I had saved the download on two DVDs, burning them from the original downloads, even though I could also have downloaded them fresh from the Amazon.com site.
I discovered that, in this circumstance, you should not try installing from a DVD backup copy of the software. Instead, it's necessary to either re-download from the reseller (Amazon.com, in this case), or, as in my case, re-create an empty folder for the Main Installer and one for the Additional Content in the place where the original folders were downloaded, which in my case was my desktop, and then COPY the entire contents from the backup DVDs I had burned back to those respective folders. Before attempting to reinstall, I uninstalled my previous attempts from the Win 7 Control Panel, by the way.
After doing that, it was simply a matter of opening the Main Installer folder and clicking on Setup.exe and following the set-up instructions, then doing the same with the Additional Content folder.
With a digital download from a reseller, following the original location of the download seems to make a difference in success or failure.
Thanks for sharing your solution, Russ!
I often find that download software is packaged within an archive (e.g. Zip, Rar, 7z). It is also the case that many downloaded .exe files contain an archive.
Pretty much everything I purchase as a digital download I try to open with 7-Zip to see if is an archive. If it is I will extract the archive to a temporary folder. I then burn a DVD from the root folder of that extract (usually the one that contains setup.exe). This gives you a DVD that will match that of a boxed retail version.
Insanity is hereditary, you get it from your children
I do the same as Neale, as it gives me a backup for the download too.
One little thing to be aware of, is that many download programs, have a folder structure, that is very important for a complete and clean install. Most Un-ZIP'ing software now does keep that folder structure by default, though that was not always the case. If given the option to Extract w/ - w/o the folder structure, choose With Folder Structure.
Further update: my "fix" worked for one day. The next day, when I opened PremEl 10, I was once more confronted by the message that the additional content was missing, even though the Windows Control Panel showed all parts of it as installed. This time, I uninstalled the additional content (all parts), then I clicked on the prompt PremEl 10 was offering (to download and install the Additional Content). I installed that, and so far, after two full days, the additional content is still there.
I don't understand why the digital download from Amazon didn't remain valid while the download directly from Adobe did. That's what seems strange to me. Maybe it has something to do with the other suggestions offered here, but I did follow the precise directions using the automatically download-and-install Amazon product. Perhaps it would be more appropriate for me to bring this up to Amazon, rather than Adobe, even though they are both involved, I presume.