The root cause of the problem is the DAT 5664 update released late last week and which incorrectly identified a number of driver files used by Windows as a potential trojan, and then quarantining them.

For many McAfee users, the results were dramatic, with users of older versions of McAfee's security software - with full subscriptions - reporting the infamous blue screen of death.

For users of O2's mobile broadband service, however, the results were more subtle, as O2 last week also updated its mobile broadband software to v2.0, adding BT Openzone to The Cloud's WiFi service that users can access as part of their subscription deal.

Because the driver for v2.0 of the O2 mobile broadband client was quarantined, the modem simply stopped working, as it was no longer recognised by the Windows operating system.

The problem hit Infosecurity's technical editor Steve Gold hard, as he was in Manchester trying to give a demonstration - via mobile broadband on his laptop - to a colleague.

"The modem wouldn't work. It was highly embarrassing, as I'm supposed to be technical editor, but the dual nature of the software interactions meant I was stumped," he says.

"And to make matters worse, even when uninstalling and then reinstalling the O2 software, the modem would work for a short while, update itself and then stop working at McAfee's software quarantined the updated driver each time."

The problem took Gold around 90 minutes to fix on Friday night and was resolved by restoring back to an earlier install of Windows XP that worked across mobile broadband - quickly uninstalling the O2 software (before it updated), then reinstalling with the software update box specifically unticked.

The good news is that O2's mobile broadband software resumes working - the bad news is that the because v2.0 of the client application is not loaded, Gold cannot use the BT Openzone network as part of his subscription.

McAfee is working on a patch for its security software. Until that ships, users of McAfee's software - who get free versions of the package with O2's fixed line broadband service - cannot easily use BT Openzone's WiFi network when out and about.

Gold says that the problem will be acutely felt by those users - who, like himself, have a fixed and mobile broadband contract with O2, and who, as a result, have free editions of McAfee's security software on their laptops.