The only secure way to wipe your hard drive is to do a full disk overwrite. This assumes that you have a hard disk drive and not a solid state drive. If you have a solid state drive you can use Secure Erase for full drive wiping, but not all SSDs have properly implemented Secure Erase, and therefor it isn't safe to rely on it unless you know for a fact that it works on your model. As far as hard drive wiping goes, your best option is to use ATA Secure Erase, there are several tutorials on how to do this online, just google for it. ATA Secure Erase uses firmware included in most modern drives, and it thoroughly wipes your drive, going as far as attempting to remove magnetic residue by putting the drive head off center during one of the passes. Another option is to use something like DBAN, in some cases I believe DBAN actually uses ATA Secure Erase, but in others it simply does a full drive overwrite without putting the head off track center. ATA Secure Erase and DBAN are the only two software programs I can recommend for doing a secure drive wipe. Encrypting a drive that has already had incriminating data on it relies on the implementation of the encryption program utilized, not all of them will securely erase previously deleted data nor will they encrypt previously deleted data. This means that you should not rely on FDE to act as a secure full drive wipe, unless you are fully aware of the details of the implementation of the FDE system you are using. Wiping files individually is also not suggested, in many cases there will still be data fragments left behind in various places.