If you do not have a Windows 7 installation DVD, you can download a legal copy with SP1 integrated from here: Official Windows 7 SP1 ISO from Digital River.
Make sure you get the same version you have, Home Premium, Pro or Ultimate and 32 or 64 bit. Note that Basic or Starter is not available.
I recommend using ImgBurn at 4X speed (or the slowest available) to create the DVD from the downloaded .iso file.
You can do this on any computer capable of burning a DVD.