Assuming you are running Windows XP, your problem is likely that Windows is detecting different system hardware and you'll need to do a "Repair Install" of Windows XP using your Windows XP Install CD. You can find web sites devoted to how to do this. I've had good luck with these, but not everyone will agree with that.
After doing a Repair Install of XP, you will have to re-activate Windows XP. And you will need to re-do any patches or service packs that aren't already installed on your Windows XP Install CD.
There are, of course, potential licensing issues here, too. If you are trying to move an OEM version of Windows, you are likely in violation of your licensing terms.