I have a serial number for PSE7, which I have de-activated on my old computer, since I want to install PSE on my new computer. However I get the message that my "serial number is invalid for this product". Is this because I can't use a PSE7 serial number for PSE11? If yes, then where can I download PSE7? I can't find it anywhere.
You can't use a serial for PSE 7 on PSE 11.
Try ebay for a PSE 7 disc if your serial is for the disc version.