It could (partly?) be due to the BackUp partition on disc.
Click whatever the Vista equivalent is of Start - Control Panel - Administrative Tools - Computer Managerment - Disc Management and look at the Disc 0 arrangement. It should show how much is in the BackUp partitiion, with the rest in HDD (C:).
Our Windows XP BackUp partitions are of the order of 8GB, but maybe Vista is bigger? HTH.