let's say you have a Dell PC , or HP or ASUS..... those manufactured computers have a windows 7 key embedded in their BIOS (which you can find on the rear of the PC or under it if it is a laptop...,

if you install an OEM version it will detect that Key use it to activate (LEGITIMATELY) and even get the theme to that manufactured PC

If you don't have OEM (Original equipment manufacturer) PC use a Pre-Activated windows it will activate your PC using a loader