This is a scam, John. The scammers, who usually call from overseas, simply cold-call people and attempt to fool them into thinking there's a problem with their PC. Think about this for a minute and you'll realise the ridiculousness of it — how can someone know what's wrong with your computer when they can't see it, and how do they know your phone number?
Computeractive magazine has been investigating this scam on an ongoing basis, and you can also read advice on dealing with computer scams in general.
If anyone does get a call of this type, the best advice is not to engage in any conversation and just put the phone down. Registering with the Telephone Preference Service probably won't help prevent future calls, since the people behind them aren't interested in being bound by this voluntary scheme.
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