|If your second drive has a lot of room, you can move files as above, and for convenience sake, put back a shortcut to the folder you moved in the original location (it only uses a few Kb space) so you can easily find things in their usual places.|
You can also uninstall a number of programs from 'C' drive and reinstall them choosing 'custom' and selecting the more open drive to install.
You can free up space by using the 'disk clean up' feature on your computer.
You can free up space by reducing the amount of space system restore uses, just move the slide to the left (say from 15% or 18% to maybe 10% or 12%) and clicking OK
You can reduce the amount of virtual memory on 'C' drive by half and activate the same amount of virtual memory usage on the more open drive.
OR, add an additional hard drive to your computer, copy your files over onto it, delete the 'D' volume, and use a partitioning program to expand 'C' drive to the entire disk area.
You have to be a little bit crazy to keep you from going insane.