you can change a few parts over time like upgrade memory or a new hard drive, but don't do it too often or upgrade too many parts that it will be a totally different machine otherwise windows will refuse to activate and you will have to call uncle Bill and explain the situation and they will tell you that since it is oem version you have to contact your pc vendor, which is really yourself because you built your own machine and they will refuse to help you unless you hand over you credit card info and they will still give you the same answer which i just explained but there is a way to force windows 7 to activate although that is beyond the scope of this post and possibly maybe against the forum rules