When you run Windows XP Setup Disc, it will prompt you on which partition to
install on, in this case either C: or D: So, just choose to install on D:
if that is what you want. This method works only when you boot from CD in
system startup, and not from your previous version of Windows.
It's a lot better to switch the hard drives; make the large one C and the
small one D. Whether you install to C or D, there will be some parts of it
that MUST be on C because it's hardcoded. So, even if you install on D,
there will still be important information used from the C drive.
Better to switch them, make the large one C, and Master, and D the slave.