Are you sure the D:\ partition is useable ?? most laptops now have a mirror image of the factory condition of the HDD on that partition in case something happens you can restore the system to factory settings and if this is the case it should not be used for storing other files.
Also how much space are you allowing system restore to use for restore points before it deletes the old ones ? VISTA has a habit of defaulting to use up to about 12% of your partition to save restore points if you do not limit it and that can really eat into the available space (so on a 500GB HDD it can use around 60GB !!) try deleting all but the most recent restore point and see if that recovers enough of your c: partition - See here for procedure to recover that space and adjust the settings