Cant activate/download photoshop cs5 on windows 10

A tip: if this was a student version it cannot be transferred to you.

Are you certain your serial number is for that product, platform and language version?  They all must match.

The original installer you received at purchase is perfecty matched with the serial number you received at the time. 

Installers obtained from other sources may not work with that serial number.

Also depending on whether it was an upgrade from a previous version, you may need the qualifying product's serial number, too.

Your account contains your registered serial numbers and products.

https://account.adobe.com/products

if you saw an invalid serial number you either made an error when entering the number OR you have the wrong installer.  details:

assuming you're using a valid adobe serial number (6 groups of 4 numbers), 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