I will try to answer all one by one.

  1. OEM systems comes with OS preinstalled and you can find the product key on a label or sticker on the back of your laptop. You can use this key anytime later on if something bad happens. Bear in mind that you cannot use this key on any other machine.
  2. DVD for the Win 7 versions is difficult to get by unless you have MSDN access. You can try getting it from Digital river or use a trial version installer if available from MS. Once again, bear in mind , the version must be same as your original OS that came with new System.
  3. If you do a fresh install, the recovery partition remains intact unless you do something with it. You can do the fresh install without touching that recovery partition.
  4. You can install Win7 OS ,same original one as many times as you like on the same system but with no hardware changes.