You can install or change hardare without re-Activating as long as you don't make any massive changes all in one go or make a number of changes across a certain length of time. Installing a new hard drive now and a new burner at a later date will not cause you any problems. If you are unsure of whether you have a legitimate copy of Windows XP then try downloading this
http://www.microsoft.com/downloads/d...displaylang=en
and you will be asked to validate your copy of XP, if it throws a problem you will soon know as it will fail to validate correctly.
There's more info on the activation process here
http://www.microsoft.com/technet/pro...ctiv.mspx#EEAA
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On another front though I would think that the installation of XP that you have on the PC is a standard install using an "generated key" or a known key that has been found on a warez site which could work for a while until MS find it's been compromised at which point the computers using it will suddenly find themselves unable to update until re-activated.
If the computer is happy to run windows update and validates correctly I wouldn't worry too much but you would be advised to look into buying a copy of XP at some point if you don't have one to hand.