The National Institute of Standards and Technology approves of only one way to securely erase data like hospital records and sensitive government documents, and that's a full random write over the entire hard disk. Writing over individual files is unsafe on a journaled filesystem, and doubly so the way Windows defragments and moves files around. An individual file can be written to multiple locations on a hard disk, and a program that "securely erases" individual files will only write over the last location. An boot disk like DBAN is your best option. It does two random writes followed by one write of zeroes to make it look like no data erasure / cover up happened. That's may seem extreme, because you have reinstall your OS, but anything less is mirage of security.