Looking at the version you mentioned, it seems that you are using Symantec Endpiont Encryption Full disk. We have disk like recovery and access utility (ISO). Recovery disk is used to decrypt the drive and access disk is to back up your data. Its a WinPE customized disk.
No Boot Situation : -
User Access Utility ISO and burn it in a CD\DVD, boot from it and backup the data. Burn the Recovery ISO into a CD\DVD and boot from it to perform decrypt.
If you are able to boot to the OS : -
Login to the SEFD client console, select the partion you want to decrypt. Once the decrypt is done, you may format the partiton.
You will find both the 'Recovery disk' & 'Access disk' ISO on the same file that you downloaded, which is SEE 8.2.1. MP3.
Note : Please ensure to back the DATA.
Let us know if you face any challenge
M_Marcos