By Dan Whitworth
Newsbeat technology reporter
Newsbeat's Dan Whitworth tries to find the best way to delete data
Chances are there's plenty of stuff on your computer you wouldn't want criminals to get hold of.
Bank details, private emails, family photos.
But what happens when you upgrade your PC and get rid of the old one?
Computer users are being warned their information is at risk, even if they delete it all.
When Alex Skipworth's hard drive broke, the company he bought it from replaced it and promised to destroy his old one.
But a few months later he was contacted by somebody in Eastern Europe.
He said: "A couple of months later I received an email from someone who said they had it and they had my bank and passport details.
"He was very clever with his words but basically he was trying to blackmail me."
There is software you can get off the internet to permanently erase files.
But Which Computing is warning sometimes just deleting them isn't enough.
It bought eight second hand hard drives from eBay and recovered 22,000 files.
Tim Ayers, who works for Palmer Data Recovery in West London, said it is very difficult to be sure no one can get hold of your old files.
Pouring water, smashing the hard drive up with a baseball bat, driving over it with a van and even setting it on fire, would only destroy the outside of it, according to him.
Newsbeat put his theory to the test and it turned out he was right.
"The one that has really suffered no damage at all is the hard drive that was run over by the van," he added.
"As for the one with water poured over it, it's pretty wet on the outside but I'm sure on the inside there is no water at all.
"If you want to guarantee destruction of your hard drive, drill a hole through it."