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Google have begun placing a yellow warning banner at the top of their search results pages for some users. Most people will never see it: the warning is triggered by a particular type of malicious software known as 'malware' that is thought to infect about 2 million computers. When you click on the warning, you are taken to a page of advice on how to remove and prevent such infections and links to helpful sites.

It's the first time a search engine has done something like this. It makes a lot of sense, since it ensures that the warning is seen by – and only by – the people who need to see it. But it may confuse some users because for many years security experts have warned against clicking on banners, ads or pop-ups that claim your computer is infected. There's a simple reason: most of the time such warnings are fakes.

Typically, clicking on one of the fake ads downloads and installs software on your computer that claims to be an anti-virus program. Instead, this program is itself malware that hijacks your computer until you enter credit card information to pay for the program. A recent study showed that these fake anti-virus programs account for 15% of all malicious software.

So, how do you tell the difference between a good warning and a bad one? It isn't always easy, especially when the warning has you worried and uncertain.

The first thing is, don't panic, particularly if your computer shows no other virus symptoms (such as running unusually slowly, crashing frequently, or popping up many unfamiliar warning messages).

Next, if you're worried and have anti-virus software installed, make sure it's up to date and then have it scan your machine. If you don't have anti-virus software installed, the WebWise guide will help you choose a suitable program and review your computer's security.

If you're still worried, you can run a second anti-virus scanner. Either download one of the free programs mentioned in the above-linked WebWise guide or go to a site such as Panda Security's Active Scan, F-Secure, or Kaspersky Lab that offers a free online scan of all or part of your system.

As a rule of thumb, whenever you get a warning on the web or via email that asks you to take action by following a link, check the link first rather than just clicking on it. One way is to copy and paste it into your browser's address bar and look at it to make sure it's taking you to the site you expect. Another is to type the address in directly or use a search engine to find the site.

Note that Google's warning obeys this general rule. The search engine does not try to download anything onto your computer. The page you reach by clicking on the warning offers only advice and links to other sites. Remember, though: if you see the warning and need to download an anti-virus scanner, do so from an uninfected machine. Read Google's .

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Wendy M. Grossman is a freelance technology writer and author living in London and is founder of The Skeptic magazine.