I poked around and found what I needed. There is a nice, relatively understandable forum called sevenforums.com with a page called Windows 7 - User Folders - Change Default Location. This is what I was looking for, to leave my profile information on C: but to move all of my data to D:. This is DIFFERENT than another option they describe, of moving your entire user profile to the other drive, in which case, you've set yourself up to have to reinstall some stuff. TAKE OPTION 2 - IT'S EASIER!!

Their tip:

Moving a user folder and it's contents to be stored at another location can be handy if you wanted or needed to save hard drive space on the C: drive.

I've set up the new external drive for backup and I'll do a backup of the C: drive before I start all of this. I can't say I've always been good about doing the backup before computer brain surgery, but I will this time because I've been meaning to get this thing out of the box for months. And if you're interested, here are some backup instructions.

SRS