Like people or saying here, you need to be more specific...
XP Tends to make you reinstall the OS, because the Hardware Specifications of your computer go into the Activation of the OS.
While newer OS such as Windows 7, you can swap all the hardware you like, and you won't need to reactivate the software.
Regardless of wether the OS doesn't need re-activating upon a hardware change, whenever you do a major hardware change (such as motherboard) you should always do a fresh OS Install to ensure drivers don't get mixed up.
I've seen problems where old chipset drivers from old motherboard on an OS get mixed up with the new ones for the new motherboard and problems/ errors start occuring.