Umm, Windows 7 is tough though since Windows stopped supporting it more or less.
You can look for the ISO installation files online. But, a lot of those aren't going to be any good. If you have no money and are careful with downloading strange files, this is a decent option.
You can purchase the installation disks secondhand. eBay usually has a decent selection.
You could also take to a computer repair shop or Best Buy and they should be able to re-install for a fee. You just have to make sure you have a product key though.
You don't need the installation disk because Windows 7 was pre-installed on your PC and it will be there when you roll back to factory settings.
I see windows 7 CDs for about $35 on the web. Some local computer stores may have it cheep too. Your only other alternative is to go into the Windows Explorer and delete all the folders other than the Windows folders.
Try f8 at boot up repair my computer look and see if it's got a built in recovery most do
f8 on startup.