And Microsoft uses a very good activation procedure. Before XP you could copy any cd. Now your new version of XP checks your hardware, asks for the license code, generates a new activation code, this activation code goes to Microsoft who gives a new activation code which you enter in XP. If everything goes well, you have 1 registered version. If you install it again, you need to go through the whole process, but
Microsoft won't give you a new activation code because your license code was already used. When you install a service pack it checks the license code, if you used a known illegal code it won't install, but then again, those service packs are really worth it to install them, if Firaxis can do the same with Civ4, there are no problems.