if i have a regular copy of windows xp professional installed on my computer, and i want to buy a new copy and change the cd key, will an OEM cd-key work without reinstalling?

Far as I know, you can only change the CD key when you activate or reactivate. Simply editing a registry entry isn't enough, because you can run into problems with the product activation code.

Could you clarify what you're trying to do here please? Is the copy you have OEM, or the copy you're purchasing?

Why do you want to change the CD key at this point in time? (If you're making an illicit installation legit, don't be afraid to say so, because that's a perfectly permissible discussion.)

The CD-key issue will only be a problem when activating, but it CAN be changed successfully.

yes, thats what im trying to do, and there is a little program that U can dl that will change the cd-key. the problem is im having some problems with with spyware and adware that microsoft has patches for, but i can't update, however i can't afford to have to start all over on this computer so im trying to fix this without having to reformat and reload it.

Use the new copy to do a refresh (over the top) installation, use the CD-key that comes with it, and all should be well. You'll need to reactivate the installation, but the new CD key should be used for generating the activation code, so it'll go through as a new installation.

Be warned, if your system has problems, an over the top install will probably make them worse, not fix them, so you'd need to format and start over anyway if that happened