I would suggest that you clone your C: drive to your D drive if you haven't already started to fill it up, then use it as your C: drive for boot purposes and use the old drive for back up.
I would also suggest that you first create 2 equal partitions on the new drive, then clone the C: drive to the first partition so that you can gain extra space and have the ability to move files from the copied old partition to the new one.
John the pilot has a good article about doing a clone. Also by doing this you will end up with a good working "BACKUP"
drive in the form of your old C: drive. Just put it safely on one side until you know that all your files are safe and have been copied over. It's always very handy to have a bootable back up drive in case something goes sour on your new set up.
if you are not interested in cloning your drive ...
to do ..
don't move files at random from the Old drive to the new .. you may move the wrong ones and leave your PC incapacitated
Don't move anything that is in Program files or Windows directories
Be careful about moving any files that you may have installed to you C: drive but aren't in the Program Files directory
some files in the root directory of your C: drive are essential for the proper booting and running of Windows .. under no circumstances should these be moved.
As I explained earlier you can move (lock, stock & barrel) the My documents folder and contents, in the manner described.
A very good file and folder management tool is called BEYOND COMPARE
and helps you not only to move files and folders across drives and directories but also allows you to do a double check that the files are identical. I use it quite a lot , especially when backing up my files since I get a real idea of what folders and files don't match or are missing ... it's a software I swear by whenever I want to compare files burnt to a CD/DVD and the original files before deleting the originals .. Too much hassle months after writing a CD and deleting the files only to find that the files are listed in the TOC but missing from the CD or unreadable .
there's a 30 day trial, so see if you like it.
I'll be around to help with any queries that you might have, so don't hesitate to shout. Just ask that you be a bit patient in case I am busy working on something else / someone else's problem.