LOST SERIAL NUMBER

Hello

My IMAC needs a total reinstall. I am a member of Creative Cloud but I also have CS5.5 on my IMAC. To do a reinstall I need the serial number of CS5.5 but cannot find it.  I only need to use my  INDESIGN files that were created in CS5.5. What can I do to solve this problem? I REALLY NEED TO ACCESS MY OLD CS5.5 files! Can anyone offer a solution??