To delete a partition using the Windows built-in utility go to Start Search and type diskpart. Please note that the diskpart utility can also be started from the Vista install media, as outlined in Bright Hub’s article Install and Dual-Boot Vista and XP on the Same Machine. That property of diskpart is useful for instances when there is no Vista installed or if Windows is corrupted. To delete a partition using diskpart:
Type list disk, then hit enter.
Type select disk X; X being the Drive on which you want to delete a partition. Hit return.
Type list partition, hit return.
Type delete partition Y; Y being the partition you want to delete followed by enter or return.
The process is depicted below for disk 0 on the test system; the letter “Y” acts as a placeholder for a partition.