For CS3, the activation servers are forever closed. Adobe will provide you with a new CS3 installer and serial number.
You will be entering your old CS3 serial number to get the new installer and serial.
Next, on the CS5 install page, You download the .exe file and the .7z When you run the .exe, it will unpack the 7z to create a folder with the installer inside.
Download Adobe Creative Suite 5 products English Universal and Standard are the same thing.
You enter your 24 digit CS5 serial carefully. If this is an upgrade, it will ask for the old CS3 serial number as a qualifying product.
If this CS5 serial is part of a Suite, then it won't work on a single Photoshop product.
Let me know how it goes.
THANK YOU for taking the time to address the problem. Out of frustration I took advantage of the current "discount" offer of PS CC for $10.00 a month while I get PS CS5 back. For the cost of an upgrade from CS3 to CS5, I get to use the CC version for a year....
I suspect that the problem all along was, as you identified, the "upgrade" product license. Indeed, I had upgraded from CS3 to PS CS5 back in 2011. Both numbers are listed on the "products" section of my Adobe account. I assumed (wrongly, I guess) that the PS extended CS5 number as listed by Adobe was the one to apply to the installer. Oh well....nobody ever guarenteed that the universe has to make sense!
Again, muchos gracias for taking the time to help me. I'll let you know if I succeed in getting either version to work again. I am ready to give up on CS3, as I didn't use it much anymore, but I would like to have CS5 back again. It was my "workhorse" for my astrophotography image processing, and I'd like it in my back pocket if I run into problems with PS CC.
If you now have a licence for CC - the desktop app also allows you to install previous versions alongside the current version (all the way back to CS6)
The idea is a thorny issue with many (rent vs own), but you get to use Photoshop on either PCs or Macs, and in any language version.
Moving from one to another is as simple as signing out and signing in.
More importantly, the OS that CS5 was happy with becomes a problem as you advance your hardware and OS. One major snag are the newer high resolution screens that CS5 and earlier cannot scale to. Another is Camera Raw if you are a photographer. v6.7 is as far as you can go with CS5, and v10 is much nicer and will cover most of the newer models since 2011. And as Dave (cm) pointed out, previous versions.
With the subscription, you are constantly fed major and minor upgrades, no more upgrade fees. You pay much more for a cellphone, electricity, insurance and so forth and it's a tiny expense compared to those.
So read up on all the new tools that will move you along quicker.