Malicious software that takes computer data hostage and then demands a ransom are increasing dramatically. According to anti-virus company Panda Software, the new technique called 'ransomware', that uses programs that prevent people from accessing their own documents, has increased by 30 per cent the last six months.

It first appeared in March with the Zippo.A Trojan horse, which searches for files such as Word documents, databases and spreadsheets, and moves them into password-encrypted ZIP files. Then, it creates another file informing the affected user that they need to pay $300 to recover their data. Since then, a spat of similar Trojans have been released, including Ransom-A, which appeared in April and asked for $10.99, and Arhiveus.A in May which asked people to buy products from an online drugstore.

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