It is believed that around 20 million users of AVG's free anti-virus software will lose their protection on February 18 because they have not downloaded the next version.

Grisoft, the company behind the software warned users of its free anti-virus package last year that the software was being upgraded; users need to download the new edition, version 7.5.

Anyone still on version 7.1 of the free software will find that it stops working on February 18, the cut-off date set by Grisoft.

The company said that of the 45 million people across the world who use its free edition, roughly half are still using the old version, and will not be protected from new viruses after the cut-off date.

This could have major side-effects, such as allowing virus writers to target unprotected AVG users and take over their PCs to be used for sending further viruses or carry out other criminal activity.

Unlike normal upgrades, which are downloaded and installed automatically by the program, the new edition requires a complete reinstallation.

A representative of Grisoft said that because the new version includes large files, it is easier for users to download it from the website.

Users have the option of upgrading to the free version 7.5 or the paid-for edition which includes technical support and some additional features, such as a firewall.

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