Users of popular firewall software, ZoneAlarm, found that they could not go online after installing an update from Microsoft last Tuesday.

The KB951748 patch was released to close a security loophole in the internet's Domain Name System (DNS). The flaw would have allowed high-tech criminals to redirect users to phishing or fake websites, even if they typed the correct address.

Reader Keith Lim found that he could not access the net on his Windows XP machine after installing the patch last Wednesday.

"I noticed that ZoneAlarm had no activity. After disabling it, I could access the internet again," said the 37-year-old project manager. These problems apparently only affected machines running Windows XP or 2000.

A spokesman for Microsoft said they were not contacted about any problems the update might have caused, reported the British Broadcasting Corporation.

Checkpoint software, which owns ZoneAlarm, has since released an update on Thursday to resolve the conflict. Users have reported that installing the 7.0.483 update for ZoneAlarm restored their internet access.

"ZoneAlarm is such a popular software (firewall), I'd expect Microsoft to have anticipated any problems with it," said Mr Lim.