how to Re-install a pre-installed windows 7 without product key

I have a Lenovo G550 model 20023 laptop, i know it had a pre-installed version of windows 7 and it was erased. I need to re-install windows 7 to my laptop however i'm facing some obstacles 1- The COA sticker is worn so i don't know which type of windows 7 was installed (Pro. or home premium .. 64bit or 32bit) 2- The COA sticker is worn so i don't see the product key that would be written...



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