I see, in that case to join partitions, you will need a third party software
like disk director, Xp doesn't have this ability natively.
Again I caution you, be explicit in your info, when you say everything saves
to C and you want to save to D, why not just manually choose where you want
to save data, ie the operating system defaults to my documents when saving
documents generally, but you are not locked in to that, you can change the
location to wherever you want ie D:
So please explain what programs your talking about and what data your
saving, and where, not just Drive C: