it should give you the option to enter a new product key. windows product keys are ez to get

if that doesn't work, then get a real version of windows like 7 or xp

or if you have to have windows 8 (lol) then you'll have to do all this:
1) make an image of your hard drive with old windows installation

2) reinstall windows on that drive

3) merge the image of your old installation with your new installation