There are several methods of achieving the results
If you have a secondary drive which haqs far greater capacity than the primary [C Drive], then far in a way the easiest would be to buy Drive Image or Ghost
Then create an image file of your C Drive into a partition on D Drive: do this in Ghost as a 'backup'
Then whilst booting up from the Ghost recovery floppy disk, 'recover' from the image file onto the Primary partition of the D Drive
Once complete swap the C Drive and D Drive around. If they are on the same IDE interface you'll need to change the swap IDE cable plugs from C to D and D to C plus change the jumper settings on secondary drive from Primary to Master and vice versa for primary drive. If they are on separate IDE interfaces, then just unplug the IDE cable at the motherboard and swap them around
Reboot the PC [now the D Drive will be the primary drive and assume the C Drive letter] and you can then delete the Windows folders from the D Drive
If you have two drives which are similar in size, it might not make sense to do the easy route. These are some more Q&As that may help to carry out your desired results
1. Is there some way to make the default install drive D instead of C? I know I can manually change it each time I do an install but I forget to do it
Yes, there is a registry entry that you can change using TweakUI t
change the location of your Program Files director
2. Is it worth doing that, since, if/when I reformat I'll have to reinstall anyway
Actually, no. It is not worth having your program files on a separate drive for the eaxct reason you gave. However, what should be on your spare drive is ALL your data
3. If no, is there some automatic way to have the programs save the data files to "D:\My Documents\My Program"? I think I can do it by manually altering the shortcut (properties, start in), but am not sure. Again I'd prefer it to be automatic as I'm both forgetful and lazy
No. Each program is different. What you can do, however, is change where your "My Documents" folder defaults to. On your D: drive create a directory called "My Documents". Then right click on the "My Documents" icon on your desktop. Change the path to "D:\My Documents". All the files in your surrent my documents folder should now get copied to D
Then it is up to you to have a look at each program and see if you can move the data files away from the C drive and onto the D.