Windows 7 may be purchased from physical stores and also as an ISO image from the Microsoft online Store.
Netbooks, those have no optical drive it is a bit complicated to install Windows 7 in such cases.
Burning an ISO image on a DVD is simple, but creating a bootable USB from a DVD installation can be complicated, Microsoft has launched its own utility software to create an USB bootable pen drive. This a small tutorial on how to use it.
Create a bootable USB drive or a DVD from an ISO image of Windows 7
The use of Windows 7 USB / DVD Download Tool is extremely simple: just specify the location of the image to burn, the USB destination (the software can also burn image on DVD) and launch copying.
If you have a boxed version, you must first convert your DVD to ISO image. Why you can use a third party program like ImgBurn when you are getting a free software from Microsoft?
The installation requires:
Windows XP SP2, Windows Vista or Windows 7 (32-bit or 64-bit)
Pentium 233-megahertz (MHz) or higher (300 MHz recommended)
50 MB free space on your hard drive
DVD or USB stick 4 GB minimum
Download Windows 7 USB / DVD Download Tool 1.0 (Official Microsoft Download Link).