Yeah, you can hey... All you have to do is:
1. Go to your start menu and right click the link for 'Computer' and click 'Manage'
2. The 'Computer Management' windows should popup. Locate where it says 'Disk Management. Once there, look at the middle, bottom column of the column.. You should see a listing of all the disk drives installed in your computer
3. Right click on your 400GB partition and click 'Shrink Volume'
4. A popup box should show up asking you to enter the amount you'd like to shrink in MB... Enter '20480' : That's exactly 20GB
5. After that click okay, after the message box closes, a black icon should be added and it should be labeled 'Unallocated Space'
6. Right click the 'Unallocated Space' and click 'New Simple Volume'... Follow through the prompts you get and enter 204840 MB as your partition desired size. Click okay and voila... You've got yourself a working partition
I hope this helped