How secure is....

I'm personally convinced overwriting once is secure enough for pretty much any purpose. I mean, if you routinely need better security than that, you should use full disk encryption.
I think using 100 x PRNG is TOTAL overkill. If you have sensitive data that requires that much sanitising then I suggest getting a new hard drive and suitably disposing of the current HD. Seriously, 1 pass should be enough but for security I would suggest that a 8 pass is more than sufficient and even then that is far too much.

Use DBAN and reinstall your operating system if you feel your security is compromised to such an extent that you feel the need to go top those extremes.
Thanks for the kind responses. The data I have *needs* to be destroyed. I'm now going to purchase a new HD and *correctly* destroy of my current one. (I've read a article on how to correctly due so. Thanks for your help.