just remember that the data WILL be recoverable IF the opposition want to spend the $$$

it all depends on WHY you want the data gone ?
I'll assume that you are giving the PC away to someone else, and do not want them to be able to see your old files ?

in this case the apps suggested will be quite sufficient ......
if, however, you are actually OBL looking to ditch data on the next targets, then not even an oxy/acetylene gas axe, followed by melting the ashes in acid will stop the feds from getting something off the HDD ......