Will I have to reinstall XP after changing the processor in my PC?

Since it's a major hardware change by Microsoft's view, you may get prompted to contact Microsoft for a new registration key.

If you are lucky then a repair install might suffice.

Probably. You'll need to reactivate and will have issues if you have an OEM licence. Don't do a repair install, there's a bug where it breaks windows update

Since it's a major hardware change by Microsoft's view, you may get prompted to contact Microsoft for a new registration key.
Most likely your system will not boot to Windows, and you will have to call Microsoft tech support, they will ask for your WinXP cd key code, then they will help you from there. But you will not need to do a reinstall.

You shouldn't need to do anything. It is possible you will need to re-enter the Windows Product Key, but this isn't usually a problem.

Even with my OEM Vista Home Basic license from Newegg.com, I was able to reformat many times on two different computers. Activation worked fine every time