If you were sent the Product Key for the Windows 7 you purchased and downloaded online then you don't need to buy it again - a retail copy is good for life and can migrate to any machine of your choice as often as you wish.
If you lost the Product Key and still have it installed, you can audit the key you own off your HD using Belarc Advisor or Magical Jellybean. Then guard it as you would other valuables.
You only need to borrow or find any retail installer for your version to install with your Product Key.
If you bought the Student Deal, MS has made good after an uproar over their fulfiller Digital River not providing disks or even an ISO file: Microsoft Fixes Windows 7 Student Edition Upgrade Problems
We can always help you get a perfect Windows 7 install here and it doesn't cost anything.