Try the instructions for DELL here
Laptop Recovery Instructions
The instructions will work if you still have the original HDD that came with the laptop when you bought it. Btw, it is assumed you have backed up your important files already.
In the recovery process, you will be asked if you want to format the partition or not. Basically by default it has 2 partitions. One is the C: drive and the other partition is the recovery partition. You can of course format the C: drive (DO NOT FORMAT the RECOVERY partition) using DBAN.
Assuming you have done the formatting of the C: drive/partition, follow the instructions on how to do a recovery using the RECOVERY partition.