None of us enjoy a site that bombards us with pop-up ads but these days, advertising poses a new threat to online businesses with programmes like spyware and adware that install themselves on your computer without your knowledge.

Recent research by IDC states that as many as two-thirds of all computers are affected by spyware. They also predict that the global cost of dealing with it will rise by 2,400 per cent over the next four years.

There are several ways you can become infected with spyware - you may open an email that is infected, visit a website that has corrupt content, or even misspell a website address and land on a site that is owned by a cyber-squatter.

But how can you tell if you've been infected with spyware? Here are some of the telltale signs.

You can't access sites you've visited before

If you find you cannot access sites you have previously visited and are being blocked by an unknown site - then this is clear sign you have spyware installed on your computer.

Your computer seems slow and continually crashes

Because spyware or adware programs are badly designed, when they run they slow your computer down and sometimes cause it to crash. If you find your computer doing this regularly, chances are your PC has been infected with unwanted software.

You see more pop ads than usual

Advertising on websites is usually contextual - so if you're on a business site, it is not uncommon to see business-related advertisements. But if you see pop-up ads all the time that are completely unrelated to the site or if these ads appear even when you're offline, it's more than likely you've got spyware installed.

Your homepage or other PC settings change

If you suddenly find you've got a new homepage without you having changed it and even though you reset it, it still reverts back - then it's a clear sign of unwanted software tampering with your settings. The same goes for additional toolbars you might find attached to your browser.

You cannot run programs on your computer

If you have trouble opening 'Windows' or running other programs on your PC then you've been infected with spyware or unauthorised adware.

If you have one or more of the above symptoms then your PC has been infected with spyware. Click for advice on how to deal with spyware and adware.