Microsoft's latest round of software updates have blocked hundreds of thousands of people from the Internet.
One of the four high priority fixes left computer owners who use ZoneAlarm's firewall without web access.
According to The Times, the MS08-037 fix is designed to plug a vulnerability in the way that Windows handles the internet's system of addresses, which could have allowed hackers to reroute web users to malicious web pages.
But this patch is also believed to have "compatibility issues" with ZoneAlarm.
A spokesman for ZoneLabs, the company that manufacturers ZoneAlarm, said the problem had come to light when US users began downloading the Microsoft code.
Zonelabs posted this advice: "Sudden loss of internet access: Microsoft Update KB951748 is known to cause loss of internet access for ZoneAlarm users. We recommend that users uninstall KB951748 using 'Add or Remove Programs' until the issue is resolved".