That question is like "how long is a piece of string?" sometimes when you do work to your PC Windows want to be activated again and then other times it just works. I think the main thing is how close to the original the modification is. If you use a similar CPU it may be OK.
I have replaced a part on one PC and had no problem, then replaced the same part on another PC and Windows needed to be activated again.
While it is a pain, a backup of your data is always recommended when doing any major work on your PC. If you have another drive the same size, there are plenty of free programs on the net that will allow you to copy an image of your drive to another and back again. (Only a few will allow from and to a different size drive.)
if it have to reactivate ,you simply call microsoft and explain the chang and they will give you a new activation key ,no need to rteinstall windows to do this .i have done this to my maching coultless times because i use to use it as ameants to test cps from customers computer ,have reactvated my winxp several times