Users said they were also unable to access their internal company networks.
Russian IT security company Kaspersky Labs told users to disable its anti-virus software or roll back the update.
Two hours later it issued a fix - but since their PCs were unable to auto-install new code from the net, users had to perform several tasks first.
Kaspersky told its customers: "Please disable the web AV component of your protection policy for your managed computers."
It then told them to go the repositories section, download an update and re-enable the protection.
The company issued a statement, apologising "for any inconvenience caused by this database update error".
"Actions have been taken to prevent such incidents from occurring in the future," it said.
Dorset-based IT consultant Graham Lord wrote on the micro-blogging site Twitter: "Bravo on breaking the internet on all your XP clients.
"Your update just set back one of my repair jobs by a day's work."
But Spain-based security blogger David Barroso tweeted: "So Kaspersky QA [quality assurance] team failed with this update but they quickly released a fix, which it is something good."
Thousands of computers running Microsoft's Windows XP operating system were unable to connect to the internet after installing an anti-virus update.