Dialers change the number used for dial-up internet access to a premium-rate number.

Dialers are not always malicious. Legitimate companies that offer downloads or games may expect you to use a premium-rate line to access their services. A pop-up prompts you to download the dialer and tells you how much calls will cost.

Other dialers may install themselves without your knowledge when you click on a pop-up message (for example, a message warning you about a virus on your computer and offering a solution). These do not offer access to any special services – they simply divert your connection so that you access the internet via a premium-rate number.

Broadband users are usually safe, even if a dialer installs itself. This is because broadband doesn’t use regular phone numbers, and because broadband users don’t usually have a dial-up modem connected.

Anti-virus software can detect and eliminate Trojan horse programs that install dialers.