Photoshop: Serial number not working (Counterfeit Product)

I bought Photoshop CS4 Extended some time ago. I installed it on my Windows 7 laptop only on the second start up of the software it said that the serial number was incorrect. It works on my old slow Windows XP and a Windows Vista laptop when connected it the internet. I had my laptop formatted and installed it again, and it worked, until i went on the internet and then it never worked again on my Win 7 laptop! I can use it on the Vista laptop but it's wireless sensor has broken so i have to keep transferring any files i need accross via external hard drives etc. which has meant i don't use it hardly ever! Any ideas?
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