If you are stuck in a situation where you have lost your windows 7 installation disk or damaged it accidentally, you can always download a copy of your windows 7 ISO file from MSDN (from Microsoft) itself. Is usual in this situation resort to pirated copies of Windows from various sites online. However, you take the risk of download pre-infected Windows (virus), files damaged and other possible problems.
Here is a list with some Windows 7 ISO (the direct links are from Microsoft servers)
Note: Sevice Pack 1 is included
Microsoft has removed the Windows 7 downloads from Digital River, but you can download them directly from Microsoft as long as you have a valid product key.
Download Windows 7 from Microsoft Product Recovery