Hiya John if you wait a little longer I am sure someone more experienced will be along to help.
From what I know of Xp, certain hardware changes are fine but if you change alot you change the hardware ID, then Windows will ask for activation, if you do not activate it ( this can be done via the internet or phone ) after 30days it locks you out of xp until you do activate it.
Now you enter the cdkey to isntall windows so let us say you installed some new hardware which changed it ( i put a slave HD in my pc and it did not bother it ) you would just attempt to activate if it accepted all is good, if not follow the phone number given and try to get a new ID from Microsoft, if you cannot, then it may be worthwhile seeing into a new copy.
www.overclockers.co.uk offer a good priced Xp Home OEM copy for about £70.
best of luck wish I could be of more help to you.