Question:
I need to delete files so then can never be recovered (undeleted). So far I
opened them as a stream and overwrote them with random data several (!) times
then renamed the file
deleted it
In some cases the files still have been recovered.
Answer:
There are very advanced techniques in forensic science nowadays. Recovery attemps may go so far to read patterns in magnetic media to reveal weakly deleted files.
To save yourself time, visit http://www.sysinternals.com and find the SDelete utility (free with source code!). SDelete implements the Department of Defense clearing and sanitizing standard (DOD 5220.22-M)
At the time of writing, this link points you directly to SDelete.
This utility works with Windows 95, 98, NT 4.0 and Win2K and comes with source code.