Click on Start in lower left and then RIGHT-click on My Computer in the menu. A mini-menu pops up, choose Manage. This will open the Computer Management window. In it on the left click to expand Storage if necessary, then click on Disk Management.
Now look in the right-hand part of this window. It has two parts, and both will scroll to show you all of their contents. In the UPPER right pane is a list of all the disks Windows can use right now. In the LOWER RIGHT pane is the list of devices connected, each represented as a horizontal block subdivided into other blocks. The one representing your 250 GB HDD unit will have a small box on its left end with a name like "DISK_1", a size of about 230 GB, and a few other details. Next to it will be a box representing the Partition you already have operating, and it will show you the size of about 128 GB and the letter name of that drive, like E: maybe. To the right of that will be another space marked simply "Unallocated Space".
To use Disk Management to create another "Disk" or Partition here, you RIGHT-CLICK on the Unallocated Space and choose to Create a Partition. I think a wizard may pop up to help you do BOTH the Partition and Format operations together, but maybe not. Anyway, I suggest you want to create a Primary Partition, set it to use up all of the available space, and not make it bootable. When it comes to options for Formatting, choose to use the NTFS File System, and I suggest you ask it to do only the Quick Format, which will take about 10 to 15 minutes. (A Full Format takes MANY hours because it does thorough testing of all of the disk, and you can choose that if you wish.) IF your system does only the Partition operation and does not include Formatting in one operation, then you can RIGHT-CLICK again on the new Partition you created and choose to Format it as a separate step. When both are done, Disk Management should show you a new drive with its own letter name assigned to that space. From here you should exit from Disk Management and reboot the computer to make the changes permanent. You should find a new drive in My Computer ready to use.
All of this assumes you want to leave the existing Partition in place and simple create another one to be able to use the remaining space as a new drive.