When you install an upgrade you SHOULD be given a screen during the install to enter the serial number of the program you had before, to validate the fact of installing an upgrade instead of a full version
This is a new PC so it has had no previous Photoshop installed. The old MacOS PS CS5 I suppose was the "previous" but it's obviously a different platform for which I have the install DVD as well. I don't recall seeing any green to enter a previous PS s/n during install. It did prompt for a s/n and I entered the one on the CS5 upg DVD case and it proceeded to install ok. After install and PS startup it pops up a window to enter a s/n or start a Trial. Enter the same s/n again and it spits out "not valid for this product". Why it said that after using the same s/n to install is strange.
Maybe I should unintall PSCS5 and try reinstalling it with my old CS3 Win DVD s/n?