Hi halparth, welcome to CDFreaks!
I suggest you take a look at SysInternals SDelete which is a free command-line utility for securely deleting files and/or free space.
From the linked page:
SDelete is a command line utility that takes a number of options. In any given use, it allows you to delete one or more files and/or directories, or to cleanse the free space on a logical disk. SDelete accepts wild card characters as part of the directory or file specifier.
Usage: sdelete [-p passes] [-s] [-q] <file or directory>
sdelete [-p passes] -z [drive letter]
Specifies number of overwrite passes
Don't print errors (quiet)
Cleanse free space