How to adjust the system partition (C:) size of Windows?

The disk management tools of Windows can adjust it to some level. But there are more space available as far as I can tell. How to further adjust the system partition (C:) size of Windows?

You may check these tools:

EASEUS Partition Master (free)

Includes Partition Manager, Disk & Partition Copy Wizard and Partition
Recovery Wizard for Windows XP/Vista/Windows 7 (32 bit and 64 bit). It
allows users to Resize/Move Partition, Extend System Drive, Copy Disk
& Partition, Merge Partition, Split Partition, Redistribute Free
Space, Convert Dynamic Disk, Partition Recovery and more. Be careful,
re-sizing partitions is usually safe but errors can happen, and always
backup anything important before modifying partitions on your hard
drive.

http://www.partition-tool.com/personal.htm

Paragon Partition Manager (free)

A good program for making general changes to hard drive partitions
while Windows is running. Create, delete, format, and resize
partitions with this program. It can also defragment, check file
system integrity, and more. Be careful, re-sizing partitions is
usually safe but errors can happen, and always backup anything
important before modifying partitions on your hard drive.

http://www.paragon-software.com/home/pm-express/download.html

Acronis Disk Director (not free; trial available)

Acronis Disk Director is a set of powerful tools that work together to
optimize your disk usage, and protect your data. Create and manage
disk partitions to store different file systems or operating systems
on one disk. Easily recover lost or deleted data, reduce time of image
backups and diagnostic tools, and improve overall PC performance.

http://www.acronis.com/en-hk/personal/disk-manager/