You have come up against one of Microsoft's clever ploys to prevent the spread of pirated or copied versions of its software across multiple computers. When the operating system is first installed it takes an inventory of all the computer sub systems. If more than one or two minor hardware changes are detected against the original installation configuration, it stops working and you need to go out and buy a new legitimate copy of the software, even though you have a technically working version and you are not trying to defraud them out of anything. It is all explained in excrutiating detail, in very small print, somewhere in the middle of the thousand pages or so of the end user licence agreement. Yeah, the one you had to tick the box to agree to before you had the chance to read it and before the computer would even start up out of the box.
Change of hard disk - new software required
Change of motherboard - new software required
Change of video card - possibly new software required
Change of network adapter - possibly new software required
Change of audio card - probably okay
Increasing memory - probably okay
Change from 32 bit to 64 bit addressing - new software required
You can't win!