If it is a Microsoft OEM copy of XP bought for installation in a new build PC, then "yes" you will be able to activate it. Different rules apply to the OEM software used by manufacturers producing PCs in bulk.
One of my PCs, which was custom built, has an OEM copy of XP. Over a period of time I have increased the RAM from 256MB to 768MB, swapped CD/DVD drives several times, installed XP in two separate hard drives and activated both (although both are used with the same hardware via a hard drive caddy) and even replaced a damaged Duron processor with an Athlon. I haven't so far had activation problems with these changes.