download utorrent on yr comp. and go to thepiratebay.org search for photoshop cs3 or 5 whatever u want...the files are large about 2gb ...if u download bundle its about 7gb..
look for the right one that says for windows or for mac..some of the links have viruses and bugs so always be careful while using torrent files.

And if the file you downloaded doesn't have serial number.. you can download the serial cracker from google.

http://thepiratebay.org/torrent/6664771/Adobe_Photoshop_CS5