PC securitySecuring your cash and privacy in the real world is one thing. Doing so on a PC is a whole new challenge. Breaking into other people's computers and doing funny stuff to them used to be a nerdy challenge; today, it's simply an opportunity to make money.

Security software companies emphasise how much you stand to lose if your PC is hacked: banking credentials, credit-card details and, ultimately, your cash. But hackers aren't the only ones getting rich; online security is big business for the good guys who sell you software to protect you from the bad guys.

In a meeting with security firm Eset in August, I was told that the firm had 80 million users. That exceeds the population of the UK - and this from a relatively small company that's barely known in western Europe. Last year's earnings were 91m (£82m); it forecasts 109m (£98m) for 2009.

A more familiar name to the public is Kaspersky, which cites 250 million users and an impressive year-on-year growth of 78 percent. The result? A handsome upward-pointing graph, and 2008 earnings of $361m (£220m).

Or take the world's largest maker of security software, Symantec, whose Norton line of antivirus programs accounts for one-third of its revenue. And what a revenue that must be. Symantec is listed on the Nasdaq100 at number 91, and has found loose change in the past few months to acquire PC Tools and MessageLabs, for $262m (£160m) and $630m (£384m) respectively. Nasdaq puts Symantec's market value today at $13bn (£7.9bn).

Income is one thing, but what about the take-home figure? Miroslav Pikus, Eset's sales and marketing director, explained with Slovakian candour that, of Eset's multimillion-euro income, almost 70 percent is pure profit.

Against this backdrop of open profiting from Windows malware, Microsoft has finally stepped up to the plate with a free antivirus package for all its vulnerable paying customers.

Other free-to-download antivirus software may offer more features. But if anyone should know how to make a security net for Windows that doesn't treacle your system or cause random freezes, it ought to be the operating system maker itself. And since Windows' weak security underpinnings lie at the root of much Windows malware, it behoves Microsoft to make some amends and give its customers the tools they need to keep the bad stuff out.

Read our full review of Microsoft Security Essentials.

[Revision: ESET has asked me to clarify that the 70 percent profit figure refers only to the head office operation in Bratislava ('ESET, s.r.o Bratislava') and does not apply to the company's worldwide independent distributor network.]