Resize an Existing Partition
With Microsoft's Disk Management Tool.

Know your Alignments. The later versions of Disk Management will make and resize partitions to the new 4K alignments, so resizing a classically aligned partition will result in one that starts with the old rules but ends on the new. This in itself may not cause any issues, but using an opposite standard of tool from existing partitions should still be avoided.Working With the New Partition Alignments

RAW and Dynamic.
Only NTFS or RAW partitions can be resized but be aware that RAW and unformatted partitions can contain such things as database files and shrinking them can be data destructive. Likewise Dynamic Disks and their partitions can be damaged by resizing and is outwith the scope of this guide. Please refer to Disk Management's own Help file if you want to adjust dynamic volumes. Don't be Dynamic

Shrinking a Partition

The resize operation may take only seconds or depending on how much data there is to be defragged and moved into the new space it may take a while. Our example here now has 208gigs of free space that we can partition up and use for other operating systems and/or a data partition, or for clones of the current Windows-7 install.

Extending a Partition

The hard drive of the partition you selected will be listed in the right hand window and in the majority of cases the left hand window will be empty (fig:8). If however you have other hard drives in the machine and you also have the correct circumstances then you may be given the option to merge free space from other hard drives to the partition you have chosen to expand. If you do have other drives showing in the left hand Available window (Hover Here) you will have the option of adding them to the Selected window. Please be acutely aware that this is a highly undesirable thing to do unless you are an expert on Dynamic Disks and understand the consequences of this action. For most of us the Next button might just as well read 'Destroy all Data on Selected Disks' because those disks will be converted to be Dynamic, which will be beyond most peoples' ability to reverse without wiping the drive/s and starting over.

So make sure that only the hard drive of the partition you want to extend is listed in the Selected window and then you can focus on the only option here that concerns us. The bottom white numbers box will show all of our available free space, but if you don't want to use all of it then change the number to the amount of MB you want to add to your selected partition (fig:9). We are wanting to increase our 80gig partition to 100gigs, so we enter 20,000 in the box

The finished result is that our Windows-7 partition has increased by the 20,000MB to make it around the 100gigs mark. If we had entered 20,480 it might show in Disk Management as exactly 100.00 GB, but these nice round gig numbers often don't show in Windows itself, so chasing them in Disk Management can be futile.