If you don’t have a disc with the operating system(Windows7) and you want to reinstall just your operating system then install with Windows ISOs(Disc Image) file is a good option rather than using a restore disc or hard drive partition that will restore your computer to factory settings.
In the computer along with windows there are lots of programs,toolbars that slows down your system.With the help of actual disc image of windows you won’t have to spend an hour removing unwanted software.
You can download full ISOs (disc images) of the various types of Windows 7.There are many ways to download Windows 7,But perhaps the easiest and fastest way to get Windows 7 iso image file is from the official HTTP server. That is you can download the iso image file from Microsoft and Amazon.com
You can download windows 7 ISOs from the following link:-
Windows 7 Home Premium x86 ISO 32-bit
Windows 7 Home Premium x64 ISO 64-bit