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New Warning
Phone call saying "you've downloaded something on your computer that is causing a problem"
In my instance they claimed to be from Microsoft security, other customers have reported that they didn't claim that.
These are random calls, and aren't to be trusted, I don't know what they are trying to achieve but don't do as they ask, and don't give them any information about you. If you want to take action check your anti-virus software is working and give it a full scan yourself, if you need anti-virus software use the free AVG and Spybot. If you google the subject there is quite a bit of info about this scam.
No-one other than your broadband provider should be able to trace your computer to you, and only your broadband provider would know how to contact you.
Some broadband providers have written to customers, these are genuine, but they don't ask you to do anything, they only report there is a problem and ask you to take action to resolve it.