While there are several methods for this task, I'll describe it for me is the most simple and fast.
What do we need?
A netbook normal.
A USB flash drive of at least 4GB. It is preferable to have one more big to also get tools that complement the installation, such as Office, LibreOffice or compression tools, for example.
A computer with Windows 7 to create the installation pendrive
ISO image of your Windows 7 installation DVD or windows, with their license and others, to take the picture.
Windows 7 USB Download Tool
Well, the first thing is to download Windows 7 USB Download Tool from this link: Windows 7 USB / DVD Download tool
Then, having located the disk image and run the program follow the instructions by selecting the disk image and destination, which is the USB flash drive connected to the computer we have.
Important: All contents of the USB flash drive to be deleted so it is important to back what is taken where appropriate.
Once the wizard is finished and you can fill the pendrive with additional programs, in my case I created a folder called extras where I've gotten several tools.
Once we have created the stick because we take the netbook, Acer Aspire One of the Junta de Andalucía, for example, to replace the Guadalinex by Windows 7 and make it boot from the USB. This is achieved by pressing ESC or F2, right now I do not remember.
When starting the installation program will have a standard installation of Windows you delete disk partitions and install the system.
When the installer asks for the key windows 7 can continue installing the system leaving it blank, if you do not have the serial number of Windows 7 at hand.
And voila, you have the system installed.
Note: A friend told me that the stick he always carried a program called Windows 7 Loader, jogging and got the serial number of Windows. I think that's too bad, but I inform you in case you take for this program to search Google ... He said he did a lot for experiments in a virtual machine for windows 7. Thing about this piece of software is that even works well on windows update so done experiments do not think anything happen.
Well, I hope I have served you help.