If you install XP then keep changing the hardware then if, and only if, you have to re-install XP you may have to re-activate it. At worst this will involve a phone call to a number posted on the screen.
IF YOUR SYSTEM IS MORE THAN SAY 18 MONTHS OLD, FORGET IT. WINDOWS XP IS VERY GOOD ON MODERN HARDWARE AND IN CONJUNCTION WITH MODERN PROGRAMMES, IT DOESN'T BODE WELL WIT PRE XP SET UPS - THAT MEANS PRE MID 2001. I KNOW, I LEARNT FROM EXPERIENCE (AND COST)
I read somewhere recently (I think it was the Q & A pages of the Jan edition of PCAdvisor) that XP will let you make changes to three of the main pieces of hardware (can't remember them all, but they did include things like sound and graphics cards as well as cpu and ram). If you make more than three changes, it will ask you simply to re-register, because it thinks you now have a new system. So, no big deal really.
and there is a question (and answer!) related to this on p. 188 of the Jan PCAdvisor. Essentially, the XP licence needs reactivating when more than 3 changes are made to the motherboard, processor, memory size, hard drive, CD drive, graphics adapter or network adapter.