Connect an external (USB) hard disk to your computer. Transfer some of the data from the C:\ drive to the external hard disk, then delete those files from the C drive. This will free up some space on the C drive.
Shut down, and turn OFF the power to the computer. Wear an anti-static grounding strap.
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Now connect the new hard disk to one of the computer's internal SATA ports, and note down the SATA port number marked on the motherboard. Next re-start, tap the appropriate key and get into the BIOS. Go through the menus and enable the SATA connector that you connected the hard drive to.
Re-start and the new drive should be visible in Control Panel ---> Administrative Tools ---> Computer Management ---> Storage ---> Disk Management. Right click on the disk that you just installed and enabled, and do a format (NTFS)
Now your newly installed drive is ready for use.