download file shredding software

[ppppenguin]

Clean wiping a disk is no problem. Free software such as killdisk will erase a disk to well beyond casual recovery. Even the spooks might well find it difficult to recover anything. Selective erasure is more tricky. Yes you can get software that will do a good job of making specific files unrecoverable but I doubt it will deal with all the temporary files and other debris that's left behind when using a computer.

Note that solid state disks require different methods to wipe them securely. Don't know enough to comment sensibly.