Hackers are exploiting Microsoft's offer of free upgrades to its new Windows 10 operating system, researchers have said.

Security experts are warning about a wave of bogus spam emails with malicious attachments, labelled as if they are legitimate copies of the new programme.

The attachments contain a "ransomware" programme that, when opened, locks all the data on a computer and demands payment to release them.

Researchers at Cisco Systems say the emails are designed to look like an official upgrade notice from Microsoft, but several words have random, out-of-place letters and punctuation.

Another important clue is that Microsoft says its update mechanism provides computer owners with a notice on their screens - not via email - when a direct internet download is available.

Experts have warned against clicking on files that come with unsolicited emails.