Open Disk management
Windows 7: Start -> Right click computer -> Manage - > Disk Management on the left panel
Windows 8.1/10: Right click start - > Disk Management.
Assuming you already have one big partition you'll have to right click your [C:] partition and choose "Shrink volume"
Where it says "Enter the amount of space to shrink in MB" enter 50000, and hit "Shrink"
Back in Disk management a 50000MB Unallocated block should show up. Right click it and choose "new simple volume"
Follow the dialog to name the new volume, and assign a drive letter