You could do that, depending on how exactly you are meaning. Legally speaking, you could not run the XP (assuming you did not mean Athlon XP, in case that is a different story) install on the new motherboard without reactivating. This does not mean that you can not do what you asked. Installing the new motherboard and CPU will do nothing without software (Linux, BSD, Windows, OSx, etc.), so you will just be limited until you install.
In case that is confusing, here it is simplified.
-Your old install may or may not boot. Serial key is invalid on the new equipment if your windows is OEM.
-Installing the hardware does not require the software at the same time.
-You will need a new copy of windows, which you have.
-Taking a break between the hardware and software installs will not make a difference.
Have fun with your new computer.
If you're in a war, instead of throwing a hand grenade at the enemy, throw one of those small pumpkins. Maybe it'll make everyone think how stupid war is, and while they are thinking, you can throw a real grenade at them.
I personally do not trust new products or webcams. This is why my desktop is 3 years old, and my laptops older than that.