Use a soft cloth.
😀
Opps, this isn't OT.
Actually, there are many freeware and shareware disk-wiping utilities. They will completely overwrite the HD sometimes many times. Do an Internet search (CNET or ZDNet is a good place to start).
However, there is no way to completely hide the data by overwriting if it is THAT sensitive from, say the FBI or CIA (other than physically destroying the HD).