A of the Microsoft Malware Protection Center offers no hard numbers on how many users might be doing this in total, but said that one recent day nearly 23 percent of PCs it had found to be infected by dangerous types of rootkit and worm were machines on which UAC had been disabled.

How UAC was disabled on those machines is an interesting point. There are only three routes starting by using a software exploit to hit a service which has administrator privileges by default, a tactic that appears to have worked in a growing number of cases.

Any malware that gets this far can turn off UAC to stop its requests for admin privileges after every reboot coming to the attention of the user.

A second method is by tricking the user into clicking 'yes' when a UAC prompt appears. A third route is to infect a PC on which UAC was already disabled, something that Microsoft thinks is now happening because a minority of users do not want UAC prompts to interrupt their use of the PC.

It is also possible for malware to infect a PC without requiring admin privileges or interacting with UAC at all, but emboldened malware writers had recently started attacking it anyway because it offers advantages in avoiding detection.