Microsoft has been accused of crossing a line and inconveniencing computer users who have yet to ditch Windows XP before it stops supporting the operating system on Tuesday.

From Friday a "pop-up" began appearing whenever XP users who run Microsoft’s free security software, Microsoft Security Essentials, turned on their computers.

The pop-up warns XP support is about to end and directs users to a site where they can find out more about upgrading to a new operating system.

A Microsoft spokeswoman was unable to confirm whether there was any way of disabling the pop-ups, other than turning off Microsoft Security Essentials, which would leave XP users more exposed to malware.

Some overseas blogs labelled the pop-ups as "scareware". XP users turned to bulletin boards to seek advice on ways to disable the "nag boxes".

Microsoft has ended support for computer operating systems before, but never one still as popular as XP.