Re: reinstalling Windows 7
The simplest method is the 'system recovery' or 'restore to factory conditions' found in the user manual of your computer. It will bring the computer in the state it was in when you bought it.
That works either by booting from a special recovery partition by pressing some function key early in the boot process, or by booting from the recovery disks that came with the computer or that you were told to make yourself.
Alas, there is no standard for this. That's why documented it is in the documentation of the computer.