Click Start, Run, type diskmgmt.msc and press Enter.
To the right of Disk0, you should see the existing partition C. To the right of that, it should be called unallocated space.
Right click unallocated space, select Create new partition. Select Extended Partition. Use all the remaining space. Click OK and so on.
Then on the graph, it becomes free space. Right click the free space, select Create logical drive. Then select the size you want (1GB=1024MB), or all of it if that's what you want. Take the default options, and when prompted, select format with NTFS. Wait for formatting to finish, until you see the word "Healthy" for the new logical drive.