Strange how inspiration hits you – or in this case, sits down next to you. As I sat on the 7.56 to Charing Cross this morning, I was contemplating weighty matters such as the price of my train ticket, which is now £1.50 more per week than it used to be, and why on earth anyone would want to daub a swastika on the inside of the carriage window, apparently with his or her own faeces. Then I began to think about what the topic of today's blog should be.

The lady sat next to me had her mobile phone in her hand and was sending a text message. She hadn't silenced the keytones, so I could hear every painful letter of it. Once she'd finished, she returned the phone right to the bottom of her handbag – just so it could fit in another couple of bars of the William Tell Overture when it next rang, which it did almost immediately.

"Did you get my message?" she said to the caller. "Yes, you see, I realise that the computer I gave you, it's still got some of my stuff on." My ears prickled. "Yes, finances, that sort of thing," she added.

I nearly screamed. Breathing as deeply as I could, I resolved to forget about the dozens and dozens of times PC Advisor has told you, dear visitor, to be very, very careful with personal information stored on your hard drives, and to tell you once more.

So here goes. Never, ever leave personal information on a system before you hand it to a friend, relative or eBay buyer. Reformatting the hard drive is the best way; investing in a data shredding package is another option. Steganos offers a package called Safe and it includes a shredder that writes over data you want to wipe more than 30 times to be sure it has gone.

Don't just put stuff in the recycle bin or right-click and choose delete – the data is still on your system and can be retrieved easily. I guarantee that 90 percent of seven-year olds could show you exactly how. If in doubt, ask someone who'll know – the IT manager at your company, or even go to our Helproom forum and ask there. Someone will give you the answer within minutes.

We've given some vendors and retailers plenty of stick in the past for selling PCs with reconditioned hard drives containing personal and other sensitive information, but please remember that ultimately it is you that has to take responsibility for it. So, lady who sat next to me on the train this morning, please be more careful in the future. Please. Please. I'm begging you.

Rant over.