which is labelled D: Backup.
How old is said computer ?
If relatively recent (to be taken generously, i.e purchased within last 5 years), then you'll probably find that the "D" drive is the manufacturers restore partition.
Mess with that at your peril unless they have provided you with restore CDs (or you have written AND TESTED some yourself).
Most manufacturers who use recovery partitions don't provide restore CDs.