A faulty patch for Windows XP cuts off a PC's network access when applied.
The fault occurs when the patch is installed and the Internet Protocol Security (IPSec) client's engine is modified to only allow secure data traffic.
If the engine cannot use ports 500 or 4500 because third-party software is using them or moderating their use, it shuts down all network traffic as a security precaution.
Uninstalling the patch using the Add/Remove Programs in the XP Control Panel restores network traffic.
Simon Conant, security programme manager for Microsoft, said: "We test on a wide variety of systems and set-ups but a few users using personal firewalls have been affected. The patch itself is a small new feature and is very easy to uninstall."
Firewall software has been identified as creating the conflict, and Microsoft is currently working on a replacement patch.
Although the firm would not say how many patches were downloaded, a Reuters report put the figure at 600,000.
Phil Robinson, managing consultant for International Risk Management, said: "From what we can tell, Symantec firewalls may be causing the problem. If so, this makes it worse - Symantec are one of the leading firewall companies.
Microsoft has also released patches for its Internet Information Services (IIS) software. Users of version 5.1 and earlier should patch immediately, as the hole allows hardware to be shut down remotely. Users of IIS 6 and Microsoft Windows Server 2003 are not affected.
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