By Richard Scott
Personal finance reporter, BBC News
There has been a rise in existing credit cards being taken over by fraudsters
More people are losing control of their bank account or credit card to fraudsters, a watchdog has warned.
Cases of "takeover fraud" in the UK increased by 75% in the last year, says the fraud prevention service Cifas.
A few years ago fraudsters who gained your personal details may have taken out loans or credit cards in your name.
But now lenders are more reluctant to give out credit. So criminals are trying to take over your existing bank account or credit cards instead.
Victims often do not even know that the fraud has taken place.
Matthew Field's computer was infected with a malicious programme while he was trying to help a friend with his computer.
"I didn't think there was anything wrong until I found that my credit card company had contacted me saying my password had been changed," he says.
"Then when I phoned them up they said that my card had been used for airline tickets around Europe."
Criminals use popular events like the World Cup to set up fake websites
The criminals often use popular stories or events and set up fake websites based around those topics. They send out emails too. When someone visits one of these websites or opens an email their computer can be infected with the malicious software.
"Cyber criminals out there are trying to infect us so they are going to all places that we go," says Greg Day, principal security analyst at the IT company McAfee.
"They'll try to hide their threats in genuine websites or set up fake websites to look like genuine websites.
"They'll commonly tie in to current affairs or activities - whether it's something like swine flu or the World Cup or the latest cricket... they're trying to make sure that we trip over their attacks."
Once one of these programmes is on your computer, it can monitor which websites you go to and what you type in.
That information - passwords and details of who you bank with, for example - can then be sent over the internet to the criminal. They can then take control of your bank account and withdraw your cash.
The number of malware programmes - an umbrella term which encompasses viruses, spyware and other malicious software - jumped by more than 400% last year compared with 2007.
But there are simple steps which people can take to protect themselves.
"There are threats everywhere," says Tony Neate from Get Safe Online. "You wouldn't dream of leaving your house with windows open or with the door open. It's exactly the same with a computer.
"You need to make sure you lock your computer. Anti-virus, anti-spyware, update the operating system - these are the things you need to do to secure your computer like you would secure your house."
A properly set up firewall is also essential. But experts argue it is important not to get paranoid - the internet can be a fantastic tool to help us with our lives, as long as we use it sensibly.