A minimum of 3 drives is required for a RAID-5 array. So you have 2 options at this point:
1.) Create a RAID-5 array with your existing 3 drives. Creating the array will delete all data on all drives in the array and you will have to do a fresh install of your O/S and programs.
Your array's total capacity will be 4TB (before formatting). After installing your O/S, install Partition Magic and create a 2nd D: partition. Then resize your C: partition to 200GB and resize your D: partition to the remaining maximum size.
2.) Purchase another 2 TB drive from Dell. You will then have 1 2TB drive as your O/S and you will have 3 drives to create a RAID-5 array.