The only real difference is a full version copy gives you Microsoft technical help for free. Thats about it. With OEM you can upgrade, add software, add hardware, replace smoked parts...including motherboards, CPU's, HD...you name it. At the very worsed inconvienence you may have is that the software reports a none-genuine copy of windows. It will then ask you to verify on-line, if it still doesn't like something it will ask you to call a number and speak to a Microsoft rep. and it gives you the number to call for you. You tell him you upgraded some major components, he gives you a new license. Simple as that, slam dunk your all set....if you don't need Microsoft technical support don't bother with the full retail. How do I know this...becasue I just build a new machine and I called MS technical support and asked these questions. SO long as you buy a lagit copy with a lagit license key, its all yours to do and change as you wish. Don't listen to the fools that think they know. It so happends after I build my new rig and reloaded my copy of Office 2010 that I had on my old machine, I accually did get this massage that my license had been used up. Told me to call a number, I did, and was given a code to activate, no questions asked. It frustrates me that people give bad advice that accualy cost others extra money. If your smart enough to build your computer and load the software, you don't need customer support, you dont need Retail. YOU WANT OEM.