in xcc, go to the 'View' menu and go to 'directories...'

there you can change the directories again.

one thing to be careful of, is that RA2 puts values into the registry about where it is located etc. The second install of RA2 might have changed these values to your d:
your first copy of ra2 might now not work properly- so you may want to reinstall again.

At that point you might want to delete the 'mix cache.dat' file, (CHECK WITH OLAF BEFORE YOU DO THIS; I am only working from memory!)