A faulty update to anti-virus software released over the weekend by Tokyo-based Trend Micro has caused PCs to slow down or stop working around the world.

For the latest news on the bug, see our update here.

By 2pm local time in Tokyo today, Trend Micro had received 311,900 calls from business and consumer users in Japan saying their PCs had been affected by a faulty data file containing information about known viruses and worms, according to the company. The data file was available between 7:30 am and 9:02 am Tokyo time on Saturday 23 April, a company spokeswoman said.

A bug in the file, Official Pattern Release (OPR) 2.594.00, created a loop that sucked up all or nearly all the processing power of PCs that had downloaded the update, she said.

Enterprises with PCs using Windows XP SP2 and Trend Micro's OfficeScan software, and consumer PCs using Windows XP SP2 and Trend Micro's PC-cillin Internet Security 2005 VirusBuster software were affected. "The problem was caused by our virus labs in Manila during the checking process when a part of the test wasn't performed," the spokeswoman said.

The company replaced the faulty file with version 2.596.00 at 10:51am Japan time. This and later OPRs issued by Trend Micro will automatically end the problem in PCs that download the new files, the company said.

The vast majority of those affected by the faulty file were in Japan, although Trend Micro has received several reports of users experiencing problems from Australia and countries in the Middle East and Europe. The company has also several hundred calls from customers in the US.

Trend Micro has yet to make any announcement on compensation or other measures for customers affected by the problem. If the company does decide to compensate customers, the details will remain confidential between customers and the company, the spokeswoman said.