You need to be sure you downloaded the same product as that for which the serial number was assigned and that it is for the same type of operating system (Windows versus Mac). Serial numbers are OS specific.
Error "The serial number is not valid for this product" | Creative Suite
http://helpx.adobe.com/creative-suite/kb/error-serial-number-valid-product.html
If you are getting an error indicating the serial is not valid, then it is not yet an issue with having multiple installations, but that could still be a problem if you ever get past the validity issue. Deleting does not do anything to deactivate an installation. To deactivate you need to either open the program and thru the Help menu choose the Deactivate option, or contact Adobe Support thru chat and ask them to reset all your activations. They no longer provide support for older products like CS5 so you should not ask for more than the reset or they will send you back to the forums where no one can perform that service.
Serial number and activation chat support (non-CC)
https://helpx.adobe.com/contact.html?step=PHSP-PHXS_downloading-installing-setting-up_licensing-activation_stillNeedHelp
As far as getting 403 errors... Make sure you are logged in to the Adobe site, have cookies enabled, clear your cookie cache. If it continues to fail try using a different browser.
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