Everything you need is here to Clean Reinstall - Factory OEM Windows 7.
You own the OS which came with the laptop for it's life, to reinstall as often and in any way you want. But you need to be able to read the Product Key, so use a magnifying glass and decipher it now. If you're missing some digits MS Phone Activation sometimes can determine its authenticity other ways to activate it for you.
Even if you Clean Reinstall I would follow Wolfgang's advice below to save a Windows 7 or Macrium backup image externally so that you have another option in case you cannot get it activated due to unreadable COA.