Gone are the days when hacking was the preserve of mischievous teenagers with nothing better to do. These days “cybercrime” is big business and the internet black market is booming.

High-tech thieves, through stealing credit card details and personal identities, now have access to more than $5 billion (£3.35 billion) in stolen funds, according to a year-long look at the “underground economy” by Symantec, the computer security company.

“In the old days you'd expect criminals to use a Ford Cortina and a stocking over their head to rob a bank,” says Graham Cluley, from Sophos, an IT security firm. “Now they're using computers. This isn't kids in basements. These are organised criminals trying to break into your bank account.”

A simple Google search can find forums where credit card details are being traded for as little as 30p. However, it would be advisable for the curious not to venture into this dark side of the web. One analyst warns that anyone who visits the sites is liable to be hacked themselves.

Meanwhile, the volume of internet viruses, the web criminal's weapon of choice, has almost tripled in the past year, according to McAfee, the anti-virus software maker. The vast majority were attempts at stealing money.

In Monster.com's case, it seems at first glance, and considering how much information was taken, that it is the victim of a database attack. This is where a hacker finds a weak point in the design of an online database, where a firm holds your personal details, and exploits the vulnerability to gain all the information held in it. Sometimes this is shockingly easy - a matter of educated guesswork and plugging in the right bit of code into a search engine.

Although companies will try their best to protect your details, you can never be 100 per cent sure that they are safe. There are simple measures you can take to avoid becoming a victim. First, try to have different passwords for different sites and change them often. Microsoft and other trusted companies often send out security updates and it is a mistake to ignore them. These updates are additional “patches” that help to build up your PC's defences.

Paying for decent anti-virus software is also a good move.

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