GParted, or the GNOME Partition Editor to use its full name, is a program for creating, resizing and removing partitions on a hard disk. IT also deals with creating and resizing the filesystems on those partitions. Although a Linux tool, and included with many Linux distributions, GParted fully understands Windows filesystems and boot methods and is available as a live CD or USB image so you can use it for partition reworking and repair on a Windows computer too.
The live disc option is usually the safest choice, unless you are working on a second disk drive that does not contain your operating system - manipulating the partitions that contain the operating system running the partition editor is like those cartoon characters that saw off a tree branch while sitting on it.
In addition to a CD iso image, GParted is available as an image file that can be used to create a bootable USB stick. Both the stick and the CD can then be used as a network boot source if you need to work with several computers on a network. GParted handles all Windows filesystems, NTFS and FAT variants in addition to all popular, and some not so popular, Linux filesystems. It even works with the Mac HFS+ filesystem, although increasing partition sizes is not possible with HFS+.
GParted is also available on other Linux live CDs, such as Knoppix and Clonezilla.
An extremely useful tool to have, whatever operating system you use.