As I suspected, your shadowstorage on D is all screwed up. Go to Start > All Programs > Accessories > right click on Command Prompt > Run as Adminitrator. Then into the little black window paste this command and then hit Enter:
vssadmin Resize ShadowStorage /For=C: /On=C: /MaxSize=300MB
Now you can clean-up D (only keep the system recovery files/folders) and set it back to it's original size - which was probably around 10GB. And as was said, do not write anything on D. If you make file backups with Vista file backup, watch out. It assumes D as default drive. You have to change that during the setup. With all the space you recovered, you could define a new partition or use an external drive ( which is better anyhow in case your disk crashes).