Persistent pop-ups, especially those of an adult nature, can be a symptom of poor online security. They are often launched by malicious software known as adware, which stows away with legitimate-looking programs that users may unwittingly install on their PCs.

File-sharing networks such as BitTorrent carry a great deal of adware mixed in with all the free music and films. Keeping to legitimate, trusted websites reduces your risk of exposure, as does following commonsense guidelines: do not click links in unsolicited e-mails, do not click on adverts offering unlikely free gifts and do not install software unless you trust its source.

Using security software that includes protection against adware and spyware as well as antivirus and a firewall — and keeping it updated — boosts your PC’s defences. Security suites are available online and in shops.

Other programs target pornography specifically. Filters such as NetNanny and Content Protect, designed for parents who want to keep children out of the internet’s darker corners, may not be 100 per cent effective, but not much gets past them.

If pop-ups do begin to appear despite your precautions, use your security software to scan the PC’s hard disk and delete or quarantine any suspicious programs. If this does not solve the problem, a company called PC Tools offers specialist scanning and removal software.

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