Is your drive partitioned to show up as two drives? Perhaps you can find a drive 'D' in 'Computer'?
How many hard drives are listed in Computer and does their combined total as shown in their 'Properties' page add up to about the right amount?
Often, Laptop suppliers partition the drive to create a 'Data' drive onto which Windows backs up any data it deems fit. This can result in the drive becoming full over time.
In Control Panel > Folder Options, you can select the 'View' tab and check the box to show hidden file and uncheck the box that says 'Hide operating system files'.
In order to delete these file, you will have to first take ownership of them.
And delete them I would. I hack Windows backup disabled in Services and any files I do want to be backed up are copied to an external drive. I never saw the point of backing up data onto the same platter as the original data. If the drive fails, the backup is automatically lost as well.
If there is another 'D' partition, I would delete it and reclaim the space for the 'C' drive.