Which? Computing magazine recovered 22,000 "deleted" files from eight computers purchased on eBay.
Criminals source old computers from internet auction sites or in rubbish tips, to find users' valuable details.
Certain software can be used to recover files that users think they have permanently deleted.
A number of software solutions exist to definitively erase files and information.
But the most straightforward solution, according to Which?, is complete destruction.
A number of recent cases have shown the dangers in disposing of second-hand equipment, from which details as well as other personal files can be retrieved.
"PCs contain more valuable personal information than ever as people increasingly shop online, use social networking sites and take digital photos," said Sarah Kidner, editor of Which? Computing.
"Such information could bring identity thieves a hefty payday."
"It sounds extreme, but the only way to be 100% safe is to smash your hard drive into smithereens."
Because those smithereens contain environmentally harmful materials, they should be recycled - for instance at the vendor from whom a new hard drive is purchased.
