Sgt_Kickaxe - 3:32 pm on Apr 28, 2012 (gmt 0)
Magnets don't destroy all data on hard drives. You need to physically drill multiple holes through the disk area in order to destroy the drive and even then, acid as suggested above isn't unreasonable if you had personal info on the drive. Some potentially revealing information is stored in the registry too.
Re-usable parts include case, video card, memory, power supply, keyboard etc. Sell them if you need an upgrade but you won't often need an upgrade anymore because processing and graphics are as advanced as bandwidth will allow. The bottleneck is no longer your computers technology, it's the connectivity.
Destroy the parts with memory, when out of your hands you have no idea who will try to get your data but I guarantee you that people who want it know where to find discarded drives.
edit: also, deleting and/or formatting a drive does not destroy the data, it just relocates it. Your smartphone is even worse for retaining data/pictures since a copy of everything you do is recorded both by the isp and by the carrier which aren't always the same company.