On a more serious note, "overwrite the existing datas with some other safe ones" is not nearly enough. The only way to truly destroy data on a disk is to write over each and every block of the disk.
There are programmes which will do several passes over the disk, writing either random or uniform data to each and every block to ensure a complete and total erase. These will require you to shove your drive on a slave connection.
However, If you only want to delete some specific files, and don't want to attach your disk as a slave (you cannot erase a disk that is 'in use' by the operating system, there is a command-line utility for DOS/Windows called
SDelete. It is a freeware tool from Sysinternals (which is now Microsoft).