You'll need to do more than just find a serial number to use because once you start Photoshop it'll try to connect to the internet to verify your copy, and they can tell when a serial code has been used more than once.
With Photoshop CS5 I had to go into the C:WindowsSystem32driversetc hosts file and add a few lines of text that would block the Adobe update site with my firewall. I used one of the random serial codes they included with the bundle I downloaded to activate it, but I didn't start the program until after I'd finished editing my hosts file and replacing the 32-bit 'amtlib.dll' file with the cracked one they provided. Haven't had any problems since.
I can PM you the link to the steps I used if you want. It's only for CS5 though.
I have a legit copy of Photoshop CS5 but was too lazy to look for my disc, lol. I'm only using CS5 in 32-bit because newer versions of Photoshop no longer support the Import Video Frames to Layers plugin, which is how I make my gifs.