Can we make or create another disk partition on computer even the operating system is already installed on disk C? How?

yes we can. Windows7 and Windows8 allows to take the hard drive space from C drive and create a secondary partition.

Go to computer Management  - > Disk Management

look for DISK0 and right click the C partition - > select "Shrink Volume"

windows will automatically calculate and give you the avaialable space for the second partition. however you can calculate by yourself and enter the space you want for secondary partition. but make sure that the remaining space in C drive should be good enough for the windows to function properly.

create the partition from extracted space and give it a logical drive letter.

I accept With Mr. Nasir Ali. If you want to more operations regarding Disk partitions and allocations and changing the format of the disk from FAT to NTFS or Vice Versa without formatting etc., you can move on with Partition Softwares like Paragon Hard Disk Manager, Easus Partiton Master, Acronis Disk Director etc., These Softwares do more than that of which windows cannot do.

Basm Allah Alrahman Alrahim

Dynamic disk is very helpful in that situation , use system spanned at windows or LVM at Linux , of course you can make shrink but make backup first because it is a very complicated process to the operating system .

Thank you for this question that help us to search and learn

From Windows7,8,2008/R2,2012 onwards hard disk partitioning can be done in Computer management and then disk management. Everything from shrinking, expanding partitions to formatting new partitions are available.

With Shrink Volume Option we can make another partition, if operating system has already installed on disk C. Go to Computer management, Then go to Disk Management, Go to Disk0, and select the C drive in which the oprating system was there. Right click and shrink volume, automatically it will calculate, select the specific volume size, and select the drive name, and finish.

Basm Allah Alrahman Alrahim

Be careful of shrinking your partition and talk a backup first 

for more detail look to my answer at bayt link

Very easy.

Control Panel > Administrative Tools > Computer Management > Storage.

Right Click on C Drive and select Shrink Volume.

A window will pop up asking you how many MB would you like to take from C Drive?

There you will also see what size will C Drive be after you remove the excess space.

I would suggest leaving C Drive to a size of80GB for Windows7 and whatever programs you are using.

If you have windows installed on SSD then don't use this feature as it will affect Drive Performance. Just get a new Drive for storage.

Once you have decided then click Shrink and wait.

After it is finished, just right click the Unalocated Disk Space and Create a new Partition. Before you use the new partition just remember to Format it.