I bought Photoshop CS5 about a year ago when I was a student. I had to reinstall after restoring my pc. I thought I had my serial number, but turns out I didnt. I origanally had to give a product code, then received my serial number. What do I need to do to get my serial number?
If you registered your product, (you have to do this, it isn't done for you) you can sign into Adobe.com, and check under your products and it should be there.
If not and the serial number was emailed, search your email archives. next idea is to contact the place you purchased the product from and hope they have a record of sale it in their archives.
I'm out of ideas at this point, I'm not sure if Adobe Customer Service can help you if you have not registered your product,but see what they suggest.