Backup everything. Once the jumpers are changed on the drives you will need
to re-install XP and other software.

Why? What difference do you expect it to make?

If you have too little free space on C there are far simpler ways to
resolve the problem than what you seem to be contemplating!

This is more complex than you seem to think.   Simply moving the files will
change the drive designations but not the registry references, which will
generally be rendered invalid.   The system may not even boot.

You really want to clone the C drive to a larger disk.. Where I am, 80 gig
drives are around $45.   You can use your existing drives if you clear the
secondary drive, perhaps to a temporary space on another much larger system,
then clone the C drive to that, then wipe the C drive and copy the D data
back to it.    After you've cloned the C drive to the larger disk, install
*only* the "new C" for the first boot.

If you don't have cloning software, there's a 15-day free trial version of
Acronis TrueImage you can download and install on another system.

HTH
-pk