The fastest option is to smash it, anything else takes hours unpon hours..
Check out a program called recuva, it recovers deleted information on the hdd. It's one of the simpler information recovery programs, you'd be surprised what the Govt has under their belt in this category.
Gutmann! that's it, it's like a 35 pass? it takes seriously a long time, it's the only way you can completely delete everything off the hard drive and make it completely unrecoverable without shashing it. A simple 3-5 pass will make it look like it did the job but like I said, the Govt has some clever ways to get it back.