Microsoft users are being warned not to open any emails claiming to contain a free upgrade to Windows 10.

The new operating system was released on July 29 but an email going around with the subject line “Windows 10 Free Upgrade” might infect your computer with a virus that renders files inaccessible.

• Why you shouldn’t upgrade to Windows 10
• Warning: the smart gadgets ripe for hacking

In the email, it appears an instalment package has been included, however that’s only an elaborate ruse for a piece of “malicious ransomware”, according to The Independent.

Those who use older operating systems have to reserve their place for an upgrade, with each mobile carrier or device type having a different waiting time.

First uncovered through a blog post by virus researcher Nick Biasini, the virus “encrypts your files, thus making them inaccessible to you, and gives you 96 hours to make a payment to have them decrypted”.

Here’s what the email looks like:

The only thing you get for free from this email, i serious technology problems. Photo: Cisco Blogs/Nick Biasini

In his blog, Mr Biasini notes that the sender ([email protected]) colour scheme and disclaimers at the bottom of the page all make the email appear real.

The email also claims that it has been passed through a mail scanner that looks for viruses.

Although if you look closer, there are errors in the messages copy and a deeper look at the sender reveals it comes from an IP address in Thailand.

Users should only use the Microsoft website to upgrade to Windows 10. To full proof their systems from any breach, they should back up their files offline.