How to reinstall windows OS?

Recently I broken my hard disk of my computer. 

Here is what I have done:

1. Download the windows 7 OS img file from campus software central website (free).

2. Install a USB flash drive for windows 7 installation:

  2.1 Follow this link: http://jingyan.baidu.com/article/d3b74d64a397631f76e6096d.html on how to make a usb installation flash drive.

  2.2 You should use a flash drive more than 4GB at least. 

3. Just replace my computer hard disk with a complete new one, and then plug in flash drive and start the computer. 

The rest will be very straightfordly follow the screen tips. If you forgret the product key, just skip it. It might ask you to setup product key later, but you can search online to get one. 

Then download the automatical driver installer according your computer brand. Then that's it.

It does not cost too much time, 20 minutes should be enough.