CS5 Design Standard serial number

An unsupported OS may not recognize the serial number.

you have a mismatch between your installation file and serial number.  ie, you can have a valid serial number, but if it's for program A and your installation file is for program B, you will see an invalid serial number message.

common mismatches are language/region (eg, serial number is for western europe and installation file is eastern europe), platform (eg, serial number is for a mac and installation file is win), license type (eg, education vs enterprise vs individual). 

and one of the most common is using a program installation file with a suite's serial number. eg, using a cs6 suite serial number with a photoshop cs6 installation file.  that won't work even though photoshop is included in that suite.  you must use the (correct) suite's installation file (and you can choose to only install one program from the suite).

here's official info about that error,  http://helpx.adobe.com/creative-suite/kb/error-serial-number-valid-product.html

bottomline: recontact adobe and let them know.